I think the LFE is to give you space. Like you said, it knocks enemies back and has a huge blast. The melee attack also seems to send enemies away.
Disc launcher is a shitty gun, but the melee attack takes up ammo instead of making you overheat. I'll probably use that for the 100 melee kill one.
Laser rifle...yeah, I don't know what the melee, but as a gun it sucks. It is one of the only trophies I don't have yet (that aren't about beating the game).
The other trophy I have trouble with is where you have to knock off that one enemies arms and legs, then melee it. It dies before I can even knock off the limbs.
The part with the Jellyfish enemies
wait, from that dark area early on?
You have like an hour to take them out before they get near the vehicle. I played on hard and the only damage the vehicle took was from those little spider bots and it was like one or two percent each time.
I mean, you're literally in a tunnel and all the enemies come at you from one direction. The tunnel is dark and the they glow bright blue to show they're coming from a mile away. It didn't even get to the point where I had to use grenades or melee because they moved so slow.