Isn't he normally a pacifist? The main argument when the Quintessence was put against Galactus was, whether or not, Phantom Stranger could be seen as a part that would intervene in the battle.
He's not a pacifist he's a cosmic neutral. In this scenario PS would fight along with the rest of the Quintessence. I agree he's the wild card since his power level is somewhere in that massive space between John Constantine and the Spectre.
__________________
“Where the longleaf pines are whispering
to him who loved them so.
Where the faint murmurs now dwindling
echo o’er tide and shore."
-A Grave Epitaph in Santa Rosa County, Florida; I wish I could remember the man's name.
Quintessence don't have that many feats and most of them are Sky father level feat wise at best. Vishanti should take it until someone can prove otherwise with few of the ambiguous Quintessence members it's hard to guage their power.