It turned out I didn't really like BTH shifting all that much on my FO Ranger, and I ended up changing that build. This build mostly plays similar to my FO Ranger except I'm just hitting stuff with melee weapons and I have a ton more MP, plus I don't have to worry about gaining negative BTH.
Against the 4th phase of the Virophage, my strategy was to inflict as much burn with heroic titan's grasp, and then modify that permanent fire burn to now deal SP damage, to keep the boss from using its SP attack and then stack some more burn. And finally I start nuking with edge of the end's burn eater skill while in beastform to shred the boss's health.
Probably didn't need to use underwyrmling but I wanted to be extra safe.
Music used:
Punishing Gray Raven OST - Industrial Strike