The time is how long you have to defeat the enemy, but they can still go through there checkpoints before the timer is up. If youre having issues where they move too much towards their checkpoints then just get up in their face, they pause to hit you and will keep doing so if youre within arms reach. I tend to melee them a lot and use shotguns on them, either way the only one I have yet to solo is the one on the mothyards, so their all very doable, but that blasted captain his shield is just so damn fast.