Before The Witcher 3 Imlerith Boss fight begins, I highly recommend stocking up on Survivability Potions such as Tawny Owl, Swallow, Blizzard or Full Moon. You can also place Thunderbolt in your Active Slot and use it to enhance your Attack Damage during the battle.
Each time Imlerith teleports, he comes back with an ice-clad armour over his entire body. This is where your Igni Sign comes to the rescue which you need to use to melt down his armour.
However, near the very end of the fight, you’ll not get time to melt his armour using Igni Sign and will have to rely on using Strong Attacks consistently.
Another thing to keep in mind is that you can’t counter/parry his mace-attacks. Stay close to him at all times and always try to dodge/roll behind him to deal some significant damage in a short while.
Moreover, always make sure to roll away as soon as you see him teleport. Imlerith’s shield is another frustrating thing during this boss fight and can knock you to the ground.
Luckily, you can rely on Grapeshot Bombs to bypass the shield and deliver a couple of strikes. You can also barge in with a couple of Fast Attacks during Imlerith’s Strong Attacks, but this requires really strict timing.
Samum is extremely useful for blinding Imlerith for a short while which will allow you to get behind him for some quick slashes before dodging away. At the very end of the fight, Imlerith will begin slashing madly in all directions.
Don’t go in to parry/counter these attacks. Instead rely on dodging or rolling and stay out of the harm’s way. As soon as you see him stuck his mace into the ground, go in and deliver only a single attacks because he will recover instantly.
Last but not the least, don’t be impatient and take your time with the fight; it’s worth it!