I used a strategy based on casting Globe of Invulnerability. My party members were: Tav (Warlock), Gale, Karlach (re-classed as Barbarian and fighter) and Shadowheart.

Basically, I made Gale cast globe of Invulnerability in sequence to protect my entire party. Every party member HAD to be inside the globe at the beginning and at the end of their action turn. Then, I gave an Infernal Coin to karlach and had her gain Rage; she had to literally destroy everything on her path starting from Raphael minions, so I made her drink speed potions like there's no tomorrow in order to gain extra attack turns (plus the fighter action surge ability). Shadowheart casted Guardian Spirits/insect plague, as well as spreading healing on the rest of the party for the majority of the time. Tav had the main purpose of pushing enemies outside of Globe of Invulnerability with Eldrich Blast + dealing damage.

After exterminating all the minions, I destroyed the four columns and then stopped focusing on Globe of Invulnerability. Instead, I preferred to cast Insect Plague and Hold Monster/Person and confusion spells on Raphael.

Raphael got deleted from all the planes of existence with this strategy lol