It's not enough to just cast your biggest spell. Even if you have time to chant it and use it, that doesn't guarantee victory.
Xelloss isn't just powerful, he's smart. And Mazoku don't teleport simply by retreating to the astral side (where the spell would still hit them) but can actually warp space itself to get out of the way. Not to mention they can break pieces of themselves off in order to escape complete destruction.
Even the incomplete Giga Slave channelled through the Sword of Light almost wasn't enough to kill a piece of Shabranigdo. That's still stronger than Xelloss, but it at least shows that you can't just call on a bigger power source and win just like that.