Though it was flames against a spirit beast that used flames, it proved to be effective in the end.
That was mainly because there was another aspect to this attack.
The blood red palm was an attack that drew on blood of those that Mo Sha had slain to make it stronger and most of the ones that he had slain were spirit beasts. This had turned his attack into an attack that was considered the bain of spirit beasts.
So the flames being infused into such an attack would naturally also have a powerful effect against spirit beasts.
That meant that it worked on this Peak Qi Condensing Realm Beast even though it was a flame attributed lion.
When the palm slammed down on the back of the Peak Qi Condensing Realm Beast, it was already too late for it to stop this attack.
The Peak Qi Condensing Realm Beast had naturally noticed this attack and wanted to block it, but it found that it couldn't control its flames.