After trying combinations of all the strats posted here, I was having no luck and getting frustrated. Until I tried Toothy!! That made the fight almost easy Toothy 1-1-2. Rip, then Shark bite then spam surge until he's dead. Then use Swamp Croaker 1-1-2 Bubble, Water Jet, Croak, Water jet until dead Mud Jumper 1-1-1 Bubble, Mudslide, Water Jet until win.
Used Hydraling 2-1-2 Ghostshell Crab 2-2-2 Left Shark 1-1-1 Whirlpool - Call Lightning - Tail Slap until Hydraling dies - Bring in Ghostshell Crab - Whirlpool - Bubble - Grasp - Whirlpool - Crab dies - Bring in Left Shark - Water Jet - Swallow You Whole - end of fight
3 Crabs with Rip > Blood in the Water works well. Only the 1st one needs Rip (preferably the most powerful one), the other 2 just Claw and BitW. Should work with almost all breed variations but, if possible, make the 1st a P/P breed for the extra bleed damage. It makes all the difference.
Turn 1. Rip
2. Blood in the Water
1st Crab dies
Bring in 2nd Crab
2+. Claw until your Crab dies
Bring in 3rd Crab
2+. Claw if necessary
This is a good tactic, but for some reason (prob breed) my crabs kept dying too early and Ruinhoof was left with like 35 health. So I subbed the last crab with Spawn of G'nathus and used thunderbolt, dive and swallow you whole. Basically time the dive so the second big hit from chew doesnt hit you.
The strat above didn't work for me. Instead, I used Gulp Froglet as first pet, applying Toxic Skin first then Swarm until dead.
THEN bring in Hydraling for Call Lightning with 100% debuff. Then Whirlpool. Hydra dies, brought in Mud Jumper (or Benax), Used Bubble, then Mudslide, and Whirlpool lands and kills him with your Benax or Mudjumper still at Full Health. Literally fool proof. Tried this strat 5 times in a row and worked perfect every time.
Hydraling 2-1-2 - Whirlpool, call Lighting, tail slap until Hydraling died
Benax - 1-1-1 - Bubble, water jet until call lightning from hydraling dropped, then mudslide and Ruinhoof was dead,
Didnt have Gulp, but didnt need it
RNG I suppose