Diablo 1 HD Hellfire Mod | Awake

I'm almost at the end of my patience about this guy.

First play-through on normal, Lvl 19 Assassin.
on Caves lvl 11, the first monster among others was this unique lightning demon called 'The Flayer'.
He's immune to everything except magic?

How do I kill him before he two-three shots me?

I cleared the ads around him and died/escaped to upper level.
He's now right at the doorway to the level, so I need to either sneak around him with Stealth skill, but he senses me eventually, or take him on toe to toe.

How do I get past him?!
Or should I just ignore him?

Could try to teleport around him and plink the demon off with arrows until he is weak enough to engage directly. Sometimes running battles are necessary.