Well, in that situation I would use tha ankylodon combined with another unit to kill the elementals faster, i keep seeing its small frontage mentionned as a negative but in most situations it's great to be so small, as it allow you to fit several chargers against the same target.
Now imagine something like an ankylodon supported by two greater fire elementals ont the side, all 3 should be able to combo charge on most units at the same time, and if later you starts to feel the "heat" from your opponent counter attacks the ankylodon might contnue to hold the ennemies while the elementals would redirect toward other targets if you feel they took too much damage.
It's just an exemple of course, I am perfectly aware that it's harder to use in an optimal manner the ankylodon, but I see potential with it. In any case, due to his point cost and versability using it to the best will certainly require thinking about how he can support or be supported by the rest of the army so that you don't waste this potential. If you use him only has a hammer, sure the greater fire elemental will be better.