Basically, on expert, I found only 1 way of getting so many kills: Spawn camp.
Build mass defences in the Nortwest corner (which is where most of their troops come from). Notice that there are lots of MGs and ATs that can be crewed by engies at the start of this mission. Im talking serious defences: 2 snipers, maybe 6-7-8 MGs, and 3-4 ATs, all pointing at that corner (best to hide in the village). Mortars are useful to, but not crucial. After the battle starts, just build as many engies as you can to recrew/repair. Placement of mines is crucial. Do not use barbed wire as it blocks AT shots. Which is a problem due to the number of Stugs.
You also cannot afford to ignore the other two roads (West and North), as anything from Stugs to flame pios and mortars can spawn here. I positioned one group of sandbagged paras at each of these (both with Recoilless Rifles), and an MG emplacement to keep any incoming troops suppressed (due to poor Para anti-inf fire with Recoilless).. Barbed wire also helps here. These ought to be able to hoold out against anything on their own, with little micro (this wasn't always true for me).
Strat. for main battle: Just make sure you keep everyone suppressed. As you should have all MG/AT guns behind sandbags, you need to keep the snipers as roving FlamePio killers, and the same is true for the MGs. They will supress em pretty quick, but if they get in range of flaming you, you're toast. You need floating engineers to keep your AT guns alive, repairing them as necessary. THIS IS CRUCIAL, because once you are down to 2 At guns, whatever arc they have, it is impossible to hold back the Stugs. I found 3 barely sufficient.
As for the rest, just try and keep as many units pinned as possible, to avoid fire for your own units. This is why you have stupid amounts of MGs --> Limit of incoming fire = limited casualties.
Eventually, it will be impossible to hold them back. At the point where it becomes obvious that a breakthrough is imminent, fall back to the Church. Hit retreat on everything. You'll lose some in the retreat, but you have to do this, because there is not time to rebuild an army before they will be on the church. DO NOT try and hold the town centre. Waste of time, coz it just gets Artilleried, and youll take heavy losses. AT THE POINT OF FLEEING, TO GET 350 KILLS, YOU NEED UPWARDS OF 250 KILLS, PROBABLY AS MANY AS 275. Do not bother defending the church if you dont get this. You wont be able to get the medal.
Fortify the church. Hopefully you have paras and engies for building defences. You need 3 AT guns for this bit, to cover the three roads. Lets hope you either are floating manpower (unlikely), or you got 3 AT guns out.. Neither of these is necessary, as it is possible to churn out the 3 AT guns (cover front, left side, right, in that order when they come out), but it will leave you struggling.. MGs do the same, snipers in the tower. You need at least 300 kills by the time the timer runs out.
Able Company enters (or is it Dog?) when the timer runs out. PAUSE. Find the exit for the troops (it is to the right of the northern road, in a gap in the hedgerow, possibly even on the east edge). Send all your tanks here. Youll probably lose at least 2 doing this, but they infinitely respawn so who cares. You need the tanks the far side of the rightmost bridge, taking care of every infantryman they see. I reckon there are likely to be almost exactly 45-50 enemy troops on the map when the timer runs out, so you need to kill all of these. Save a tank til last (preferably disabling its engine in the process). This will ensure that you dont accidentally miss someone or anything.
When I completed this level, I got EXACTLY 350 kills, and there were no more on the map, so check everywhere..
So yeah. To wrap up. Camp Northwest corner with MGs and ATs. Use paras and MG nests to cover over roads. Mines are crucial. Snipers and dead pio-teams are also crucial. German mortar teams are annoying, but more dangerous to try and take out than anything. You need 275 kills ideally before you retreat to the church. You need AT LEAST 300 kills by end of timer, preferably 310-315 to be sure. Move tanks to exit zone. Kill all troops. Leave tanks til last.