for a boss pet I found a REALLY good combo. since you're going for the achievements I assume you have a decent amount of gold to spend.

also assuming you're a vet/old school player.

acquire a dovabear: chance to heavy bleed for a dot, reduce hit chance, armor, and crit.

use the aa from Kelvin or George

and use whichever pet you want for happiness. I'm a tank so I chose stampy for the big boost in HP, and damage reduction. 15% damage and 10% crit is also nice!