I had to put my PB KunLai Runt in the 1st slot, take a hit, then forfeit to get his health lower. I believe it took him down to 1200. Any old pet battle won't do it because it takes 10% from ALL the pets, not just active pet. Once I did this, the strat worked on the 2nd try. (Got a 900+ crit against my imp the 1st try and made me fail)
Shadeflicker phase shifts on the first turn Corefire Imp is out every single time. Order of Corefire's attacks ar e incorrect. Tried starting with immolation but still not successful. Regardless of damage done with first attack, Shadeflicker was consistently killing Squirky one turn before Void Gate came off coolodown and therefore kill Corefire with Gate before it could attack. 25 tries, no success.
If you don't have the right breed for the 3rd pet bc the HP is above 1400, you can cheese it by joining a pet battle by one of the many pets in the area and just forfeiting so Kun-Lai/Yeti take a 10% hp loss (only Kun-Lai/Yeti and randoms must be on this forfeiting team), then just slot them into 3rd spott, and Corefire w the higher hp will be forced in on round 1.
I did this w a Kun-Lai Runt P/B breed and it went exactly as described above without a hitch.