I finally completed the game and got all 8 achievements.
The Chronosceptor did feel a bit OP on the Campaigner and I used that as a final blow weapon. It wasn't much of a challenge, though it was on Easy. I just used the Minigun, seeming that does pretty weak damage for a weapon that shreds through rounds at the most rapid rate.
As for a bug report:
Thunder got stuck on this part before I killed him. I was farming respawing ammo and that's when he got stuck. I capped this at the time. As you can see... I came prepared with armor, even though I didn't really need it. xD The enemies in the fortress do lots of damage, but I'll have to player harder the next time.