Log on mage, enter the dungeon for your level. Option 1: fight one group at a time by pulling with fire bolts, shooting down enemies as they come. Use AE as melees swarm. Clean up with more fire. Option 2: aggro multiple groups and get enemies to cluster. Use AE attacks while chugging potions as needed. Clean up with more fire. Option 2 works better with melee enemies, and it is more interesting as option 1 is mostly holding down the left mouse button.
Still, there is only so much fun you can get from this. It does not vary. The attacks are a few options with different colors for different types of damage. Fighting red rats at level 20 is much the same as fighting brown ones at level 2. Each dungeon does add new models, along with new versions of old enemies, but skeletons with plasma rifles fight just like rats with flamethrowers. The only changes of pace are enemies with annoying abilities (heals, stuns) and champions, who are the same enemies with more hit points. Increasing the numbers does not change the fundamental.
Four dungeons into it, the boss fights are nothing special. Sissirat is my favorite so far: the first boss and a huge fight with poison everywhere. Algor is just a melee bruiser. Vergrim was annoying and frustrating, constantly floating around while summoning exploding rats (that knock you down for three seconds. Each). Widow Maker is a big spider who summons little spiders, who also roots and debuffs.
I think I got all the gameplay in my (less than) one month of play. I may return at some point when I feel the urge to kill ten rats.