an easly way to kill him is with ice traps [glyph of warding - level 3 spell that cleric, Wiz, sor can all cast]
for best results you want one character to drop a bottle of water next to it... this is a free action
then have any character target the bottle... spore druids or any one using two hand cross bows can do this as a bonus or reaction, now our target is wet he takes double damage
if you have a warlock then drop a dex hex with is a bonus action giving disadvantage on dex will speed things up but its not a game breaker
have Shadowheart, Ghal and yourself drop ice traps for 6D6 ice damage, half that if he pass a dex save then double what ever damage remains... two ice traps will normally do the job but finish with any an icebolt if you need to
the staff of bless can be cast before the combat to add a free 2D4 to hit rolls, the staff of Ice can make ice bolt hit harder [both are got in the area and if your Warlock does his blast then turn off any push back because you want the bullette to stay in the ice puddle as that gives a good chance it will drop prone if you can't kill him in the first round of combat
btw this same trick works well against
Bernard because ice traps are not aan attack spell until the target goes hostle i.e, you can drop a water, then drop a trap and Then start the battle with icebolt or Hex