Ok, if u have a saved file that leads u to the last lvl, then this will work, firstly, before loading ur game, go to multiplayer, and go to game options and then select the pigs health to half, this will half all pigs health in EVERY game, once ur done with that, go and load ur game, for this to work, u must have like 3 hero's. start the game and when ur 1st pig starts, quickly, run to the enemy pillbox and use jetpack, but not too close, when the jetpack is above the pillbox, eject the jetpack and the jetpack should hit the pillbox 20. u then should have like 10-5 second left. stand back and use a cluster grenade, make sure u hit the pillbox ABOVE the pillbox, giving the pillbox a 30, and possibly 5 20's. The pillbox should have less than 100 hps. then the enemy's turn... Once the enemy did his turn, its ur turn again, this move should be tricky, run to the pillbox again and use another jetpack and do the same with the jetpack, but this time, use airstrike, and use it, making the end (or the front of it)pointing north-east from south-west, but dont put the crosshare on exacly the pillbox, make sure none of ur pigs are there, then, FIRE!. once fired, the pillbox should explode, the enemy pig will have really low health (since its a gunner) and its gotta fall into the poison water and should drown.

Yes, this is really long but i can assure u that this is possible, enjoy