Yea, I think you're talking about Heigan...
Anyways, here are some ideas for upped difficulty (heroic,nightmare):
- Have a turret randomly fire an AoE slow on the ground (anyone in it has reduced speed by 25% or something)
- Spawn an additional add with the repair bots that needs to be killed next to the turret: (when the add dies, anyone in a 15? meter radius takes an additional 5-10% more damage (stacking, but can drop off))-this requires a tank swap every so often so their stacks don't get too high.
Those are just a few ideas off the top of my head.