Playing level 200 ep8 with ~70% damage reduction and 24k hp, my experience is the following:
[ul][li]Immolation columns will OHKO.[/li]
[li]Explosion mines will OHKO.[/li]
[li]“Crouching” fire blast will OHKO.[/li]
[li]“Jumping” fire blast will OHKO.[/li][/ul]
The “Jump 'n Slash” reduces me to about half hp, and the “Spinning” attack generally isn’t a problem.
To adress the question, no, I don’t think Ignis is too strong. Each of the “deadly” attacks you have ample opportunity to avoid. When he casts a ring of explosion mines, plan an exit route; there’s always at least one gap. If he starts to back up, or crouches down before the mines have gone off, get out of there. I guess this is the general strategy for most “end bosses”: don’t get hit.