This achievement takes a bit of time and luck to get. But overall it only took me three attempts to achieve. This achievement is probably best to get solo or with experienced teammates. I did it solo.
The weapon I used was the Toxic Needler. It does a large amount of poison damage over time and heals you with it's heavy attack. The more enemies your heavy attack hits, the more HP you get back. It does take some getting used to, and you do have a very low amount of HP.
Since the first dungeon is the easiest, I did this one. Make sure to restart until you get the vampire mode buff. There is also one that decreases enemy damage.
I was able to earn 7,500 gold and upgrade the first two skills completely, and the first tier of the third skill. This gives you more critical, more HP drain, more ammo and more DOT. Make sure to open every chest and container as they give you passive power-ups that stack and last for the entire dungeon. Some of them, like shield, are extremely useful.
Make sure to kill most enemies. Run from shielded enemies and they can't be hit with ranged attacks unless you are inside their bubble. Some of them you will be required to kill, so go inside their bubble shield and hit them a bit for DOT to take, and then run away from them and the other minions that will likely be chasing you. Shielded enemies are the caster who summons minions (they drop 100 gold per kill) and lays down an AOE damage spell, and the knights with giant swords.
The two mini bosses of this area aren't even an issue, just run around them in a circle and whittle them down.
The final boss is a bit annoying. The first form is easy to beat without taking damage. Kill the eye minions as soon as they spawn with normal attacks, while jumping over the shock waves before he hits the ground with his weapons. You need to be directly in the middle of both of his weapons to avoid both shock waves. After 5 hits, he will let them sit on the ground so you have a chance to break them. Make sure to break them at the same time so you don't have to deal with a different shock wave pattern. If you take damage on this phase, you can use your heavy attack to heal by using it on his weapons. The best thing to do during this part of the fight is not get hit.
His second phase is the actual annoying one. You have to catapult onto a ledge and damage him before he puts an AOE damage effect on the platforms you can hit him from. You can't damage him from anywhere else. While this AOE is preventing you from remaining on that platform, you must dodge giant blocks that will instantly kill you if they hit you. The lower his HP, the more waves of blocks you have to dodge. I found that the best place to avoid this is at the bottom near the gate, but I am not sure if it's 100% safe. Once that is done, and the platform is safe to stand on again, use your ultimate (1.5x) damage and hit him with whatever you need to. If you hp is low, use heavy attacks, if not, use normal attacks for more damage output. You should only need to repeat this a few times to beat him. Or if you stay up there and spam your heavy attack heal you can heal through his aoe damage.
I'm not sure if wands are any better, but I don't think any of them heal you without having to use your ultimate. Best of luck. Stick with this and you'll get it eventually. I hope this wasn't too much to read.