Ok, so I will warn you before I continue, this is a horrible, seemingly quite buggy piece of DLC.
1. So when you begin you will have a few waves on enemies coming at you from the front. First thing you want to do is go to the chest on your left which has infinite health packs and ammo, you will need both. Then crouch behind the boxes, move from left to right and spray their general area with your Minigun for a few waves. Don't worry about ammo as a 500 clip will cover you for these waves.
2. The second part is a few snipers, they are easily identifiable on the left, right and one at the back by their green lasers. As you probably know with Metro games, hit detection can be a bit pap, but just look for their green lasers, sit back and cut off the view of the others while you concentrate on them one by one. The weapon to use is a pump powered rail gun which is awesome and again, found on your left next to the ammo and health cache.
3. The snipers will continue to spawn when the third part of the mission comes along. Much like the end of the campaign, the guy with his shield bitches will appear in front of you, and he needs to be stopped before he reaches you. Don't worry about hitting him right off the bat, instead concentrate on taking out the remaining snipers on the left and right, as they will cause you no end of grief if not dealt with. Unfortunately, they spawn randomly so I can't tell you specifically where they come from. But once you have taken them out, switch back to the Minigun. You will probably need some ammo by this point, so go to the chest on the left and pick up the ammo.
With the shield guys, you can just smash the shields in front of the main man with the Minigun and spray the big man when he is revealed, or you can use the sniper rifle on the shield guys, then the Minigun on the big guy. Either one works and in general, you can even give him 30 seconds or so before you actually need to start working on him. In honesty, he doesn't seem as tough as the guy at the end of the campaign.
4. So once he has been dispatched, then come a few more waves in front again. Same as before, just get the minigun stocked and dispatch them with a similar method.
5. Now comes the worst part of all - the tank. It will appear on the right hand side on the tracks, and the guy standing to your right will offer you a grenade launcher type weapon. TAKE IT!
Unlike the SP Campaign, you will need to just fire at the thing, rather than shooting specific parts. So just load up the grenade launcher (3 shots at a time) and keep firing at it. The tank has two parts to look out for:
A. The Turret which is fairly harmless if you keep moving and
B. The rocket guy on top which can be a nightmare!
Just work your way around in a constant anti-clockwise circle: Shoot 3 shots when on the right, get ammo medi kit from left, heal (if needed) and reload when you are at the back - rinse and repeat.
The rocket guy will fire rockets towards the back and occasionally (and this is the annoying bit) he will fire rockets underneath you that kill you in one hit. These can be VERY annoying! Do not think you can hit him through the gap on the right whilst standing still, as he will rocket you easily. Also, don't sit towards the back and pop in and out, the key to beat the randomness of his rocket fire is to KEEP MOVING in the anti-clockwise pattern. I cannot stress this enough, it will take a bit more luck than judgement, but after persisting I managed to get it.
I tried all sorts of methods and the keep moving around in a circle one and popping out to shoot worked best.
The floor rocket glitch (or at least it seems to be glitched) can really frustrate but keep at it, eventually you will get a run where you manage to avoid his random floor rocket shots!
He probably takes about 30 grenades to die, so keep moving and good luck! Once that is done the achievo will pop and you can never worry about this horrendous piece of DLC again!