Much more pleasant battle after the stone rush nerf. With 1 round CD it's a lot easier.
Starting with Baby Ape (breed #4) vs sludge: start with charging up barrel toss. Then use roar (+25 % dmg), then barrel toss (hits for about 1700-1800 oneshotting him). This is overkill because of roar hitting him, so you could of course use rake instead of roar to reduce his next hit. The baby ape then fights the earth elemental, who stuns and then kills him; putting both his stun-abilities on cd.
Switch to Snarly or Eternal Strider (breed #8). You will kill the earth elemental before his stuns are available again. Eternal strider: pump then cleansing rain for dmg bonus, use water jet until he dies, or water jet then pump again. Snarly: use rip then blood in the water and finish him off with surge. Your pet will then face off with darnak.
If you have snarly, use surge (or water jet). Darnak will then burrow on the next round, meaning you won't get another hit unless you use surge, but with reduced dmg because of stoneskin on Darnak. If you use Eternal strider and killed the earth elemental with water jet, you can use water jet on the first round vs Darnak, then pump to nuke him (Strider is faster than Darnak, so you get to hit him again before he burrows). When he is burrowed, use pump again. If he misses you, you get to nuke him with pump once more.
Darnak will then use stone rush and probably kill your pet. You switch to Wild Jade Hatchling. The Hatchling uses call lightning (bonus dmg vs Darnak). Finish him off with tail sweep (which gets bonus dmg on both hits from lightning storm, which has bonus dmg vs Darnak as a beast). You probably kill him before he burrows again, but if you don't kill him until then, use lift-off.
This made the fight super easy. Got the earth elemental in the bag too, wee