In case you do not know how to feed on the Inferno's monsters, here it is.
First, click on the Inferno Cavern on the ground, then Choose Transfer Monster.
So, assume you have the required monsters already (20 Spurtz and 10 Zagnoid), simply transfer the correct amount of them (make sure the housing capacity is not exceeded).
Finally, Click 'Transfer Monsters' again. When the inferno monsters are residing on your Monster Housing on the ground, then only you can feed your champion on them. 1st: Click on the Inferno Cavern on the ground to Transfer Monster 2nd: Click the correct amount of the required Monster