The good news is that Tau are probably the most fundamentally broken army in the game, which works to your advantage, particularly against Tyranids.
Pulse rifles wound gaunts on 2's, and are AP5. Gaunts should not be an issue at any time.
But that's not really the question, was it? The ion cannon-armed hammerhead is actually going to be more useful against Tyranids than the more common railgun-armed configuration. Multiple shots will kill Warriors, which are synapse creatures and thus targets of some priority. The hammerhead should be quite safe from Tyranid fire, and you're doing something wrong if it gets assaulted.
Hopefully you've got a few Crisis suits armed in the proven Fireknife configuration-- a missile pod and plasma rifle as its armaments. While really intended to shred Space Marines, a Fireknife can do a number on monstrous creatures as well since its weapons are high strength.