Strong minions are your best defense.
Beside that, knowledge.
That of the Arena maps/monsters AI. Tactical thinking. Don’t stand in the open, where monsters can jump at you from afar. Put your minions ahead, hide around the corner, run in circles, whatever works works. It feels like some spawns are scripted (same monsters appear in same sequence). If you can recognize a familiar setup, you know when and where to expect a dangerous mob/pack.
Craft better gear.
These are numbers I currently have:
~400-500% increased minion life
~700-1000 minion dodge
~400% minion damage (+200% from 50 INT) - more always better but I’m still working on it.
100% glancing blow chance - best defense you can afford.
zero dodge rating for you - just don’t have enough affixes on gear for that.
1.5k life or 1k life with 500+ all resists. Tyrant passive (+60% life to you) helps.
Spec your minions to be durable. IMO Skeletons are best specced into melee only, max health leech (take Necro passives as well), max bleed chance (they can leech from bleed), max dps (more leech). With these amounts of leech my skellies were keeping themselves healthy at wave 150. Minion Dodge helps, as it increases minion’s effective HP, without making it harder to leech to full life.
Golem - I use spectral, as it gets 500 dodge bonus, which makes him really tanky. He still dies cause he can’t leech as effectively as Skellies, but it takes time. The more time it takes the better for you.
Wraiths - usually die from old age before they get killed, so just make them as annoying as possible. Summon 7 at once, to save time for more pressing matters like running arount trying to not get hit.