I like Alexander’s idea. The human mission consists of both soldiers and noncombatants, the latter being necessary to perform critical technical tasks, so the soldiers have to escort the noncombatants around.
Destroy the alien device (eg. a beacon or a computer nexus). It should be easy enough to model a grey alien device or several as some kind of smooth-surfaced, nondescript, inscrutable-looking thing.
Prevent the aliens performing critical technical tasks at various specific locations across the board. Maybe they’re trying to set up several smaller devices as part of some array.
Rescue scientists or civilians that have already been abducted.