I cant survive the first arena boss

First of all, this is a fantastic game. love it. ive made 20 characters and then deleted 10 to make 10 more so it has got the right stuff.

However, I just can’t get past the first arena boss. As soon as he enter im one shotted and i dont even get to see anything.
I’ve tried this with many many different builds that blast through the lvl62 monolith with ease and i have 75% all res and I have some armor and stuff but he immediately kills me no matter what.

I’m sure there are some defensive thresholds I need to meet but I don’t know which, but im thinking maybe that first boss is Too hard compared to the other contents. Just from a newbie fairly bad players perspective.

How do I survive the arena boss?
What kind of stats am I looking to get before entering?

Note that I’ve tried with all classes, both minion build, melee and caster builds.

You mention your resistances but this is far from being the best/only layer of defence you need.
How is your life pool? Armor? If you are ward build, how much ward do you have? Do you have critical strike mitigation?

Edit : Résistance is LE are not as mandatory/punitive as PoE for exemple.

Crit avoidance/ crit dmg reduction would be my first guess. Bosses have different crit bases I believe (much higher chance) and taking a crit from these guys is rough.

I generally use T4 Arena to test new builds since its a good mix of clear and single target and less costly than dungeons, less variance than echoes.

Also note that all the 3 bosses have a kinda oneshot ability that requires fairly high(unreasonable for most builds imo) defense to ignore (Rogue has the acid flasks you can’t stay in, Primalist has the auto tracking avalanche which is honestly overtuned and Sentinel has the telegraphed smelter’s wrath).