This level is very frustrating, but I did notice one important thing; it's all about checkpoints and knowing where the crappy AI spawns! If you get stuck on a bad checkpoint after the first two guns it's best to start the whole thing over and try to get a better checkpoint. Doesn't seem logical (you finally got the checkpoint) but it'll save you a LOT of time! It's faster to do the whole level again and get a good checkpoint, than try to get to the last two guns with a bad one.
I noticed that getting into the little building where enemies spawn is easier when you can get a checkpoint where your tank moves up til the point where it gets destroyed. To get that, YOU need to move forward after taking out the second gun (the one on the left).
First time I did it, I used the panzershrecks to get the first gun on the left and the second on the right, but on my second time they didn't work so I did all four manually. It doesn't matter much, just do the gun on the right side first as shown in many videos, then move to the tank in the middle for cover and a panzerschreck for the second gun, if it doesn't work you'll have to rush the second gun. Make sure you throw molotovs to distract the guys at the back for a few seconds and take out the second gun. Then move FAST to get a good checkpoint. You'll need to move forward, but if you do it here you'll get enemies and grenades all over you so get back to the righthand side of the tank in the middle that had the panzerschreck next to it (the whole right area is clear now anyway), move around it from the right and then move as close to the enemies as you can get. This should make your tank & guys move up until the point where your tank gets destroyed, and you should get your checkpoint while they are moving up. Because you got it here they'll move up everytime you die, giving you more cover. As soon as they start to move, you have triggered them so you can get back to a safer spot. If you aren't fast enough you'll get a checkpoint with your guys still further back and you will have to make them move up by moving forward and you'll have to do this every time you die. This alone is tricky and then after making them move you still need to time it just right to get into the little "spawn building". You won't get another checkpoint until all four guns are taken care of, so you need to make sure you can make it into the little building pretty easily (let's say at least 1 out of 4/5 tries). Having the checkpoint like this allows you to just rush for that little building every time you die, making it a lot easier!
If you have the checkpoint you can prepare to rush the little spawn building. Make sure you have enough ammo (two main guns, a rifle + submachine or two submachines will do just fine, I preferred to have an MP40 + PPSH, but one of these and a rifle is good enough!)
When you rush the building don't use grenades! There are enough grenades flying around your head, plus you probably don't have time to cook them anyway and throwing them without cooking them is just plain stupid. So use your molotovs again (two should do it) and RUN! Don't care about the guys that are there, because as soon as you get in the building you should be safe. It's possible that there is one guy in the building, or one blocking the door. Shoot or knife them and relax once you're in. The others won't chase you once you get through.
Make sure to move to the back of the building (by the big hole in the wall) where you can see the two last guns, and DON'T GO BACK. No matter what, don't move past the little wall that you'll see in the middel of the room until you have shot everything you see by the last two guns, cause as soon as you do that an enemy WILL spawn behind you, so be prepared.
Now, shoot everything you see on your left and right side by the last two guns. Some enemies will come up and shoot you, but it's nothing compared to the rain of grenades and gunfire before, so relax and keep shooting until your men move up (you'll see them on your right by the gun after a while). Don't move until they've moved.
Once you see them it's time to go back through the door. It IS possible that there is still one enemy out there waiting for you and like I said before at least one enemy will spawn behind you as soon as you go back out, so move fast and turn around to shoot the guy behind you. Make sure everything is safe (shouldn't be anyone there) grab some ammo from the bodies outside en move up to the third gun. Kill the guys that are still there, and take it out. The move back again and climb the stairs up the building you were in before, but grab the panzerschreck on the right of the stairs first.
When you get up there fire the panzerschreck at the fourth gun. If it doesn't work you'll have to do it manually, but if you take your time and move from cover to cover while shooting enemies in between you should be fine. When you destroy the fourth gun stay low and wait for your checkpoint... The hard part is over now!
For the rest of the level you should be fine, just read the other solution. I preferred to get into the building in the very right though, but I don't think it matters much, both buildings have a PTRS sniper rifle. Good luck! :)