your current strat is an 9+ turn win, assuming it works, people are reporting it can fail. I'm not sure if what i did is perfect but it was a 6 turn win, as follows:
Nether Faerie Dragon 221 (mine is P/P)
2. Arcane Blast
3. Life Exchange
4. Arcane Blast
NFD died, swap in chrominius
6. Surge of Power.
Nice strat, thank you. Don't have a lvled-up Spawn of Onyxia so finding something that I could use was a relief. Gnasher was left with 81hp after Surge of Power, so had to finish it off with my third pet. Good thing I put in another lvl 25 just in case! :D
This one worked, changed it up a bit. My NFD died too early, so I went to my 3rd pet (spawn of onyxia) before going to chrome. For pet two I did liftoff, Healing flame, tail sweep until dead. Brought in Chrom - howl, surge = win.
lil deathwing (or anything with a CD) makes a good second pet before chrominius, also worth noting that if gnasher isn't under 407, then you should spend a turn or two using bite before surge of power. (or howl is he's under 675)
Used Yu’la 1 2 2 Celestial Blessing swap to Nether Faerie 2 2 1 pass Exchange pass pass pass until dead Bring in Yu’la celest exchange celest until dead Chrominus 1 1 2 howl bite then surge. by Kananga
Worked first time for me - Chrominius should only get hit by 3 rips, which won't come close to killing him. If he's taking a maul hit (Around 800dmg), then your timing may be off with the previous rounds.
Had a similar issue. Those occasions where chrominus died, My whelp had done well and lived to round 6 or longer. Switching after 3 tail sweeps (when spawn of onyxia is predicted to die) fixed the issue and chrominus worked fine.