Mini Thor - This guy is pretty awesome. His health pool, combined with the fact that he's Mechanical (he revives when dying once per battle), gives him quite the advantage in terms of durability. His abilities are also pretty solid.
Minefield is excellent against opponents with several pets, and
Sticky Grenade can do a lot of damage if you're in for the long haul. Their alternatives are also quite good, with
Toxic Smoke being a decent DoT and
Launch Rocket being able to do a lot of damage in a shorter timeframe than the grenade.
The only weak ability on this dude is in my opinion the
Batter ability. Due to the Thor's lack of speed, you will not be faster than your opponent for the majority of battles, making the
Missile the safer option.
But overall, if you're lacking a tank in your pet team, this one is a safe option.