Thing was with the mecha, it was a huge target, not very mobile, and had limited weaponry. By keeping its attention constantly engaged and disarming it, we kept it from singling out and obliterating a target. A big Grimm is more likely to move around constantly, to be unpredictable, and generally more versatile.

So the mecha, while deadly and hard to damage effectively, wasn't hard to take down once we had a strategy going. This was almost exactly the case in the series when team RWBY took on Roman in the huge mech suit.