This is my codex of accumulated knowledge regarding effective tactics in Exterminatus. It's probably a work in progress. Some of these tactics may also work well vs. human opponents.
In general, the most optimal kill order FOR ORCS is
2) ranged boyz
3) ranged nobz
4) melee boyz
5) melee nobz
This is because you can simply run away from the melee orcs and completely neutralize them, but you can't run away from the shooters. And you don't want to fight melee while you're being shot at, though you can kill the shooters while avoiding melee. Also, the boyz should be targeted before the nobs because they have a more dangerous damage/health ratio. Most boyz seem to die with 1/5 the effort a nob takes to kill, but the boyz seem to 1/2 the damage a nob does. THE ONE EXCEPTION to this is the Bonus Round, primarily because of those damn Plasma Cannon Chaos guys. In the Bonus Round, it's actually better to draw the melee Chaos (the red daemons and the hammer guys) to your position in cover, kill them there with AoE and Meltas, and then use hit-and-run attacks to take out the Plasma Cannon Chaos, leaving the Bolter Chaos for last.
The AoE stun attacks are really quite powerful. Especially if you use a perk that boosts them like Sure Strike or Furious Stomp. A lot of people don't seem to use them though. Stun attacks with a Thunder Hammer and Sure Strike can win you 90% of your fights. Please note that the AoE stun for swords doesn't always fire. It seems to require a few conditions be met first:
1) If you have Sure Strike and Swordsman's Zeal, Sure Strike should be the 1st (top) perk and Swordsman's Zeal should be the 2nd (bottom) perk
2) That the 3 standard attacks you need to chain before you get the double-stop option must hit, but not kill, eneimes. Just to be clear, the sword's AoE stuns come from this chain: Attack, Attack, Attack, Stun (should give the big blue stuns), Stun (seems more damage than stun)
There are only 4 things that will ALWAYS interupt/knock back the Nob Bosses and Chaos Hammer guys:
1) Grenades (Frag for sure, maybe the Flash ones too)
2) Assault Pack landings (powered landings, not gliding ones)
The only thing I know that will actually stun (like, the bluish/purplish aura stun) nob bosses is:
4) Thunder Hammers + Sure Strike IF you land BOTH stun attacks (1st the single-target and then the subsequent AoE one)
They will almost always ignore your standard stun attacks, even if you have stun-boosting perks like Sure Strike or Furious Stomp, so don't rely on them. In addition, the Chaos Hammer guys can be kept in a stun lock by a Thunder Hammer provided you get the first hit in and then just keep hitting them with the standard attack. This sometimes, but rarely, also works on the Orc Bosses.
The Power Axe may be the least effective melee weapon (well, besides the knife). It doesn't make up with damage what is loses with speed (and the corresponding reduction in healing if you use the Zeal perks). The Hammer is best, but the swords work well too on the early maps that have mostly unarmored orcs.
When in doubt, use this:
First Few Arenas
Assault Marine, Thunder Hammer, Plasma Pistol, Frag Grenades, Sure Strike, Impenetrable
Make sure that Sure Strike is your first/highest perk, because there's a bug that will sometimes prevent it from working if it's the 2nd/lower perk. Spam your stun attacks. The first one is a single-target stun, but the 2nd one in the chain is a huge AoE one that not only stuns almost everything but also does a lot of damage. It's the simplest way to be pretty effective in any of the arenas.
Last Few Arenas
Tactical Marine, Plasma Gun, Melta Gun, Frag Grenades, Weapon Versatility, Improved Melta Charge
IMO, the Tactical has the best survivability, primarily due to the ability to roll and sprint. With this build, you should use
- melee or the pistol to take out Squigs, Gretchin, and Slugga/Shoota Boyz
- Plasma Gun to take out Nobz and Skar Boyz at range
- Melta Gun to take out Nobz, 'Ard Boyz, and Skar Boyz when they get close
2-3 Bolt Pistol shots, 3-4 Plasma Pistol shots, or 3-4 knife hits will kill any shoota boy, slugga boy, or flash git. If you're a Tac or Dev playing with other shooters on your team, help conserve ammo by using melee or your pistol against these weaker orcs.
If you're using a Tactical, try to bring a Melta. They're very useful weapons, probably the single most useful Tactical weapon. They're effective vs. armor, they have an AoE attack like a shotgun, and they stun/knockback everything, including the heaviest melee guys. Because they work like shotguns, they also require less aiming, which is great given how many fights are vs melee swarms.
In the first arena, you will not encounter many armored orcs. The one exception could be the shooter nobs. But other than that, everything's very squishy. Bolt weapons (Bolt Pistol, Bolter, Storm Bolter, Stalker Bolter, and Heavy Bolter) are very effective against Health but not that effective vs. Armor. So, in the first Arena, bolt weapons are fine. The second arena tends to have a balance between armored and unarmored orcs, so bolt weapons will still work okay, but you'll notice a performance hit. By the third arena, you really shouldn't bring any bolt weapons because they're just too weak against all the armored orcs that will be thrown at you. So please, for the good of your team, if you reach the third arena, please be sure to bring any of the following:
- Plasma Pistol
- Plasma Gun
- Plasma Cannon
- Power Axe (or better yet)
- Thunder Hammer
Also, don't bring flash grenades. Bring frags or stims. The reason is that frag grenades and stims don't hurt your teammates, but flash grenades do, in that they blind your teammates too, not just the orcs. Plus, there are plenty of other ways to stun a mass of orcs that DON'T also stun/blind your teammates.
Try not to die at all. You may not succeed, but just trying is what's important. If you set that as your goal, you'll learn to play smarter. It also helps your team by not wasting lives.
Learn where the tactically advantageous locations are on each map. These locations usually have two characteristics:
1) They're on high ground. This usually grants you cover from shooters on the lower levels.
2) They usually can only be attacked via no more than 2 narrow paths, which help you focus your defense and act as choke points ideal for AoE stuns, frag grenades, and Meltas.
Examples include the bridges in the 2nd and 4th Kalkys arenas, the central hill in the 1st Hab arena, the 3-story building that you can glitch up in the 4th Hab arena.
If you're already 41, keep the younger ones alive and let them get more kills. The EXP is wasted on you, but it helps them get stronger and be more useful.
The Bolt Pistol has a governor on it that limits the Rate of Fire. The Plasma Pistol doesn't have this governor, which means you can fire as fast as you like, but the game doesn't seem to register all those shots if you fire too quickly. It may just be caused by lag, but whatever the cause, be sure to pace your shots with the Plasma Pistol or you'll just waste ammo.
Arena 1 - the hill to the left of the spawn point is the single most important site on this map. A single Dev can draw and kill off 2-3 spawns worth of orcs by themselves if positioned at or near the top. A pathing bug helps with this by tying up the orcs who try to get you. Just keep your eye out for shootas as they don't suffer this limitation and are your single biggest threat up there.
Arena 2 - The spawn point is the best place to fight from most of the time because it's the best choke point on the map. Orcs can only come from one direction and you have cover on both flanks.
Wave 7, a single Assault Marine can hold the porch of the building on the left if they use a lot of AoE stuns.
Wave 9 has lots of squigs spawning from two points in the building on the right.
Arena 3 - DON'T HIT THE WEIRD BOY UNTIL THE FINAL WAVE (15)!!! This map tends to bug out a lot, and I think I've narrowed down the cause to messing with the Weird Boy. If you manage to kill him before the last wave, you'll definitely bug, but it seems even just damaging him too much will prevent him from appearing to trigger other set-pieces in earlier waves either, which also bugs you out. So, just don't touch him until Wave 15.
Wave 14, cap point A first (the one on the ground level) because there's no cover there and it's much harder to hold later in that wave. Everyone should cap A first, then move to B. Point B is the one upstairs on the opposite side. There are tons of shooters on this wave, but you can take cover from all of them by simply standing behind either of the two pillars flanking the point, and you will still be close enough to contribute to the cap. Just watch out for the melee rushes. On the plus side, the melee rushes can only get to you after navigating a staircase, which makes a great choke point for frags, meltas, plasma bombs, etc.
Arena 4 - There is a glitch on the staircase of the 3-story building to the right of the spawn point that can enable Tacticals and Devs to get to the 3rd floor where almost no orcs ever spawn and where no orcs seem to come up to get you. So, if you like sniping or bombarding, this is a good vantage point for you. The temple at the far center end is the best place to fight, particularly on the staircase. The staircase acts as a great choke point, funneling melee guys to a narrow path (great for AoE attacks), and so long as you're actually on that central staircase, the building provides cover from shooters. If you need a place to fall back to and to limit the disadvantage you have in numbers, that staircase is it.
Wave 19 has 3 capture points. In general, try to cap all 3 at the same time, putting 1 Marine on each of the two side seals and two on the middle seal. Devs tend to be terrible at this because they're too slow, but if possible, put Assaults on the two side cap points and Tacticals on the center point. The Tacticals are best at rolling and melting everything around them, which is probably the most effective tactic for capping that center point.
Arena 1 -
Arena 2 - Once again, the spawn point is the most strategic position as it funnels all the bad guys down a single path and provides some cover on the flanks. Because the vast majority of orcs are melee, and the shooter orcs that do spawn have lots of obstacles in front of their spawn points, a single 41 marine could do this arena by themselves, so there aren't really any special tricks needed.
Arena 3 -
Wave 15, there are a bunch of squigs. If you're in a bind, have 2 marines cover each end of the bridge on the upper level. there are only 2 ways onto that bridge, and even 1 marine could hold each end since squigs die in 1 shot, so that's the best way to defend against the swarm. Just be careful, the very last spawn of squigs comes from the tube that's in the middle of that bridge.
Arena 4 -
Wave 17, the seal is on the bridge. This is one of the best positions on the map because melee can only come at you from 2 directions, down a narrow path, and you have some cover from shooters on the ground.
Wave 18, the seal is on the ground closer to the spawn point. The key to this round is to suppress the orc shooters that appear on the bridge above you. Tacs and Devs can shoot them, or Assaults can take turns jumping up there, killing a few, and jumping away. The trouble for the Assaults is that there are a ton of shooters on the far side of the map who can hit anything on the bridge, so if you go up there, you won't be able to stay there for long. I've seen a Dev on the spawn-point-side of the bridge, in cover, keep that bridge clear of shooters before, so you may want to try that.
Wave 19, the seal is on the opposite side of the map. If an Assault Marine stands atop the pipe that acts as a pathway near the seal, and if everyone else is on the seal, the orcs (who are dumb) will simply move to the inside of the pipe and stand there. The Assault on the top of the pipe is the closest target to their spawn points, but they can't get him, so they just chill in the pipe, waiting for the seal to be capped and to get fragged and melta-ed to mush.
Wave 21, the Bonus Wave, has those damn Chaos Plasma Cannons again. The best position to fight in is actually right by the spawn point. There are 2 broken pipes that stick up in the air, and the area around the base of them provides perfect cover. If you need a place to fall back to, or to fight the melee guys without simultaneously being shot at, that's where to do it.