Context: I've completed all of the story quests of the new version up to Diva Must Die. I've been able to summon the Slime Diva, but it keeps killing me. Not even leveling to level 10 and unlocking the Fighter Class has helped.

Anyone have any tips or tricks for dealing with the Diva?

The fight's okay, I got it down to half health with a level 7 novice using a sword I got off a greater wisp, just gotta dodge sometimes and make sure to block if necessary, I guess.
If you need something that's a bit of a cheat, the game is coded weirdly and most attacks scale with click speed way more than it first appears. I've got an autoclicker built into my mouse and using it triples my attack speed for some reason (minus with hammers.)

Other option is to farm all the other stuff and hope something good drops, but the RNG is pretty bad so IDK.

Buy a bunch of healing items (which are very cheap) and spam them if you're health gets low

It's a boss, so play carefully, ranged weapon (bow or scepter), is ofc the easiest hit while circling her constantly and as soon as she does an attack stop attacking focus on getting away from it, but ultimately that is still by just move sideways from her in that constant circling.

If you must use melee well for the same reasons as above, never be close to her for more than a few seconds to avoid be surprised by an attack, this means that as far as weapon choice harder hitting but slow weapon (ie hammer) is your best bet. Right after she does an attack move in hit once, maybe twice if you are daring, and get the hell out untill she does the next attack, rinse and repeat.

I would say imo it is a little annoying she despawn if you do die to her, having to go and kill SlimeK's yet again to get her to spawn again

My strategy with her was just stick and move. Easiest method for ranged damage is to just stock up on rocks and use the "throw rock" skill as that does a consistent 5 damage to her. As someone else has said you can only really hit her in melee once or twice between attacks, as she can smack you away when she uses certain attacks.

If you just want to fight her at range the little outcrop with the tree that you can reach by jumping onto the part of the pillar and then jumping to it will make it so you don't have to deal with the tar spawns, but it will make avoiding her attacks a bit harder with the small amount of space you got, especially the aoe attacks like the bouncing slime ball and the slime eruption.

It's as simple as dodge > hit > repeat,  really.

She pretty much one shots unless you have good armor so you might as well just play defensively.

Her minions are the worst part, I personally recommend using the hammer with its jump attack unlocked.

Level 10 fighter with an iron axehammer (dropped by the pigs) with the associated mastery (hammer spin), and healing and rage at the ready. Lots of kiting, Leg Up whenever possible, kill off the minions when behind cover. Every once and a while I'd activate rage and take 1-2 jump attacks through her, then return to kiting. 

Finished her off with a rock.