...

I am imagining, later in the RP, Marik being able to use a laser/plasma sword and duel an alien, or a new plasma sword wielding alien.

If this were an actual X-Com game, I'd think of kinetic strike used by mecs, particularly the animation.

Berserker lunges on top of you? BOOM, PLASMA SWORD TO THE STOMACH.

Mechtoid? SLICE AND DICE.

And for the regular aliens like sections, like how Kinetic strike has a standard punch animation...
Decapitation animation.