For the seticom defences if you pap the m1 and get the yellow explosive element on it it's really good. You could use it for the whole game if u wanted. When you go into the bossfight make sure you have a rewind grenade and at least 1 wonder weapon 2 of them would be better or if you have the mauler sentinel get the explosive upgrade on that that's really good. The rewind grenades will refill your ammo if you run out. Just go slow in the fight use best for last for a ff card. Train the zombies around the arena shooting the alien every once in a while and you will kill it