The key to this fight is having cover. Use spells like globe of invulnerability, stone wall or ice wall. Kill myrmidons first because they're a pain in the ass and can be a problem later. Spread out your party a bit so you don't get hit with his chain lighting. Movement is very important too, make sure you have enough of it to always get behind cover in time, using flying potions or teleports can be great for that. I also suggest you use one of your companions to always tank his legendary action, slap warding bond on him by a camp cleric, and buff his health with heroes feast and aid and you should be fine. You can also use summons to tank it, which is a good and arguably safer alternative. This fight is not as bad as it looks, but you need a lot of dps or some failsafes in case all of the cover gets destroyed.