I use a FWW+PB Najasaurus for most champs. Highest AoE of any pet, and they're pretty tough. Completely and utterly immune to Poison, so you can always heal them (other than Mortal Strike), basically immune to debuffs/paralyze with 170+ Resist, and fast with 190+ DEX. With Consume Damage running on them, they're basically unkillable for most champ spawns. Only downside is, they're not ridable.
If i'm doing Dragon Turtle Champ in Eodon, i'll swap out my Najasaurus for a RC+AP Frost Mite at Lvl 4 spawn.