McTavish has melta bombs, if you can get near enough. But honestly, you probably don't need to worry about destroying it. The Tau aren't going to send an unsupported hammerhead into the city to pursue you, you can go flat out to get out of range and use the urban terrain to avoid line of sight, and the few shots they get get off can be jinked to avoid. The hammerhead is a threat, but only if you try and face it head on. Which you should not be doing, because in case you've forgotten, the rules we are using are called Covert Operations.
You seem to have forgotten that you're meant to be using stealth for this. Your plan for this mission consisted of immediately opening fire on some of the various sentries, alerting them to your presence and forcing your troops to get into a firefight just to survive, which alerted more sentries. Oh, and having Maria toss a grenade into a building full of fire warriors. End result? You gave the Tau 4 klaxon counters on turn 1, and they all came running.
If you had taken the time to hide in the garage and ambush the guards in melee, you could have taken them down at most one klaxon counter gained. Or you could have had half your force follow Hertius, while the rest locked the devilfish hatches and had the translator bluff the guards for a few turns, saying the doors were jammed. There's probably other options I haven't thought of. It was possible to do this mission without ever even seeing the hammerhead.
