Griever's pretty easy by this time you should (doesn't mean you need to) have 255 strength on squall, that combined wih aura is an easy way to slaughter all of ultimecia's forms, if you don't have 255 strength let me recommend going out one of the gates into the normal world and drawing around the island of heaven and hell or using the devour strategy. Just having good junctions is all you need (unless you're ultima shadow).

Now, the cheap way: Use holy wars, hero or the end.

Main danger is Shockwave Pulsar so try to have at least 9000 life. Ultimecia is very easy if you just renzokuken every chance u get