Ancient Idol and Rot Beard are best, since they do true damage, and it can hit every hero on Invokus.
Snap is good on the lower levels, since his white is true damage too, he can clear crippled enemies for good.
For electric damage bring Cyclops/Rabid.
For toxic - Hydra.
For necrotic - Dracul/Void/Rev.
Only Water damage is somewhat tough, use Frosty/Rot/Wyrm/whoever you got with water damage. But even if you bring none, new creeps will be summoned after some time, and water resist hero can just stay until the end of the battle, it won't matter.