I used mimic to distract them (any summon really) and then rained down Meteorite of Astel using the temp infinite mana potion which took half their health or more.
That or Comet Azur for smaller/single target work great on slow bosses.
Well I think my game is working as intended and you've managed to reset RH by triggering the invasion after the burning?
The invader is because Gideon All Knowing and his minion were harassasing the Albinauric Wolf Lady in Liurna for her two medallion pieces, because it unlocks Haligtree which is where a Great Rune boss is and they want the runes to become Elden Lord. Since you kill the minion, Gideon I guess decides to work through you instead.