(let's do a little story)
Aliens invade the earth, they evolve in those type of huge invincible monsters (tyrant, stage 3). The humans, to protect themselves of the tyrants, create the Tesla generator (stage 3) which can easily kill a tyrant.
If we follow this, this would mean that aliens have a week point with electricity. Now, Advanced Marauders can zap people, why not make it so that if a Tesla attack is directed onto them, the electricity is redirected to an enemy building or person. And for the Marauder, why not making Tesla proof? Since the marauders are resistant to Tesla charge, they should be extremely week to bullets. For other energy guns the damage would be diminished and the reste of the energy would be converted and transformed into electricity to zap people
Hereunder is a list of the weak/strong points for each alien (tell me if I'm wrong).
Creature

Weak point

Strong point
Dretch

not much health

everything (a little bit), fast
Granger

slow, not much health

everything (more then dretch)
Advanced Granger

slow

everything (less the granger, more then dretch)
Marauder

material (very weak)

energy (no damage)
Advanced Marauder

material (extremely weak)

energy (backfires to enemy)
Basilik

energy

material
Advanced Basilik

energy

material
Tyrant

energy

material
What I mean here by "material" are guns with profectiles (machinegunes, rifles, turrets, etc.). What I mean by "energy" are guns which use energy like the Las Gun, Pulse gun, lucifer canon.