I thought Mini Thor did well against them too. But the reason for that was because he could chain up his damage.
Beasts have the passive that makes them hit harder when they are under half health. So, the Mini Thor can apply the DoT, then sticky grenade, then hit a couple times with his regular rocket attack. The idea is get work them down to half with a couple hits, then when the sticky grenade goes off, they are only alive to use their passive once or twice. That is the problem with repairing. You can't do anything for a couple turns, and if a beast with it's passive up and an anti-mechanical attack is working on your mech, that's a bad thing. It will kill your mech before the repair completes.