Apparently worms are a bit stronger than I had first thought. I think our final upgrade had them doing around 70 damage each, every time they spawn there was 10 of them and each worm could split into 3 more worms if they managed to kill something. Yeah I think I'll be upgrading worms more often in the future.
We managed to hit 30 minutes for the first time, which was our goal this run but we still are getting obliterated in the 6th map and not quite sure how we are going to get over that hurdle.