So it’s a 20 second channel.
In lava.
Go resto, strip off all your gear that gives you stamina.
Damage from the lava is based on %, so stamina just makes you take more damage, while secondary stats only increase your healing throughput.
Hop in, blow a convoke, barkskin and frenzy regen for the bonus healing, click the shell, Based on the convoke RNG you should get low, then the talent for automatic frenzy regen will go off and keep you alive until you finish the cast.
You’ll still die afterwords, but you can solo the snail mount challenge.