uniques are pretty tough... as tempting as it can be to build pure damage, since this is a roguelike, you should be focusing on not dying above all else.
early game it's suggested to focus on things that boost your raw hp, maybe even put some of your spare points into constitution instead of your secondary stat if you feel you need it.
later game, go for % damage reductions.

you like archmages? they can be very tanky, but in a bit of an unorthodox way. instead of having high health or defenses, they focus on shields. the fourth talent in the arcane tree is a permanent health shield, and a pretty huge buff.
earlier on, temporal shield is very good. it's basically just a free shielding rune, you'll probably end up using it the whole run.

debuffs are probably the easiest way to die, even if you're relatively tanky. as a magic character, "shatter afflictions" is the best rune there is. it doesn't take up your turn, removes up to three debuffs AND gives you a shield.
since it's magic, you won't be able to use it when silenced... but don't worry! you can use a wild infusion that cleanses mental debuffs, and you can get rid of that no problem.

if all else fails and you absolutely cannot win, there's nothing wrong with running. blink runes and movement infusions are big helps. as an archmage, you have both phase door and teleport unlocked by default. i strongly suggest using these if things start looking grim.

The discord perm invite is https://discord.gg/tales-of-majeyal