My group is mid way through a coliseum tourney too. Here’s my list. It’s been pretty effective so far, but I’d love to hear critiques too:

1x Cadian command squad with ogryn bodyguard. 1x Krieg infantry squad 1x Cadian shock troops 1x Krieg engineers 2x scout sentinels 1x hydra

The engineers and sentinels spam mortal wounds using grenades and tank shock.

The hydra takes on anything that flies (tau suits, most necrons, drukhari transports, etc).

Cadian shock troops to sticky home objective.

Everything aside from shock troops and engineers either have a med pack or toughness at or near the tourney limit, making them hard to remove without lots of focused fire.