I think in S1, they should clear the immediate perrimeter, so when they hit S2, they don't have to loaf around killing dretches and baselisks till they hit S3 and the aliens hit S2/S3 too.
Humans should have the main corridors occupied for entryways/waypoints to the alien base, and let the remaining corridors for surpise attacks for the humans, that way, the pressure on the human base will be far less because most aliens prefer to take the easiest way to the human base, and the defence structures will take care of the remaining corridors/hallways which aliens could pass by.
I, for myself, mostly take the lasgun and the light armour in S1 and clear the immediate peremitter from dretchs, baselisks and dragoons, whilst in S2, i take the light armour, helmet, jetpack (sometimes) and the lasgun and: 1. if the rest of my team is camping, my objective will be to hold out as long as possible till they get knocked to their senses and go attack (trying to tell them is 9/10 times useless, because they mosly don't listen),
or: 2. whilst my team is clearing the area, i take the fastest way to the alien base and try to damage the alien base as much as possible (A.K.A suicide run) before my teammates arrive to finish the job.
PS: in S3, if i got the credits, i take the battlesuit and lasgun (offensive, because it's ammo is enough to sustain an offensive run) or battlesuit and chaingun (defensive, because the ammo is gone too fast) and try to wretch havok to every alien around the map
Anyways, that's just my two cents, i hope some one has use to my sadvice :wink: .