Just finished a game in Fortress mode on Cataract. I was playing the Unclean Beast and my partner was a Torchbearer (fire mode .. it matters), and our opponents were a Regulus and an Oak. The beginning of the game was tough, but we outplayed them, got several levels ahead, and were on our way to dominating the map and making a final push against their fortresses.
Then Oak hit level 10.
From then on the game went like this: Oak respawns at base, charges at fortress, attacks, we kill him, he becomes invincible and does more damage, and keeps attacking our fortress.
We tried killing him near his base, we tried pushing straight for their fortresses, we even tried getting a Universal Gadget and healing the things, but we weren't able to counter the fact that Oak, by himself, with a decent amount of armor, can take down a fortress with no consequences. Yeah, we killed him as soon as he showed up, but ten seconds of free buffed attacks against a fortress kills it in a couple passes.
Is there any counter to this? It just feels utterly unfair to have an invincible opponent destroying the thing you need to keep safe. Maybe if the structure armor/HP upgrades were worth anything I could use those?
If the map was larger you could ambush him on his way in -- but we tried that and he just ignored us; by the time we killed him he was halfway to the fortress and he just ran a few more steps to finish it off.
And to add insult to injury, the guy playing Oak wasn't even very good. I've played Oak lots of times and I know how to play him; this guy was using Shield at the wrong time (which is what gave us some early kills and let us turn around a kill deficit), not using Penitence, not using his minions well...but it didn't matter because he got to level 10 and knew how to commit suicide repeatedly, which is apparently what you do to win.
Meh.