for the nexus strike as soon as you drop down into the nexus area do a 180 and you can get in behind the big stone slab, pop out either side and throw rockets and icebreaker shots to the face. for the sepiks strike if you climb the stairs to the left as soon as you enter sepiks lair you can hide behind a red colored crate thingy and hit sepiks from pretty much any spot on map, then just duck to reload or avoid his attacks. only shanks will come up there to get you. there is a slim chance sepiks will teleport on top of you, so you need to run fast to avoid his close quarters attack, then just head back up when he moves away. for the archon priest fight if you head back up the cliff you drop down to archons lair you can then jump up to a higher cliff and have a good shot on the archon while being fairly protected and out of sniper range