This was a purist run on Creeplight Ruins. I was playing as a Goblin Assassin with the intention of getting the Unstoppable badge. This run started out very smooth then hit a major hiccup towards to the end, namely that I explored the
entire map before finally locating the boss.
I did get lucky and find the Gloves of Midas (an item that you
always want to find on Purist runs), but the shops were pretty barren. I ended up only buying a tower shield, so I didn't really need them.

As a low-level assassin, I'm completely uninterested in fighting monsters that are equal level to me. I'm looking for a level 2 monster to start earning some bonus XP. I want to completely ignore the cultists; they're a lot of effort and worth very little XP. I find the CYDSTEPP glyph in the corner of the map as I look for an appropriate mark.
I luck out and find a level 3 Bandit. Bandits have very low damage, and their only real threat is that they reduce your resitances. As an assassin, I have no resistances, so I can easily kill him with some hit and run with poison

Took a few tiles to pull off, and I utilize my death protection to end this a little early. I've also located a level 4 berserker in the process. He'll be my next victim.

That wasn't too hard at all, and I notice a level 7 Berserker in the corner. It's a bit aggressive for my level 3 Assassin to take him on, but I'll go for it.

The level 7 bandit deals more damage than I have hit points, so conventional poison hit and run won't work. Instead, I elect to build up burning damage using the BURNDAYRAZ glyph. I find both Binlor's alter and a shop that sells Midas' Gloves while I'm doing this; great finds each.
It looks like I'm ready for a one-two knockout with my death protection,
but I'm not. The combat prediction here is
wrong. When I attack him, those 10 points of burning will go away, and my next attack will actually deal 10 fewer damage than is predicted. I need to build up a little more burning first.
After finishing the level 7 bandit off, I quietly attack and defeat a level 5 bandit he's so weak I don't even need to use poison). There are no obvious enemies left to attack. I'm now level 5, and the only revealed monsters that are higher level than me are zombies and cultists. Yuck, neither are good targets for an assassin. I worship Binlor to build up piety while I explore.

I very quickly encounter a level 9 Bandit. This run has been one big smorgasbord of bonus XP from bandits. I have 55 HP and he deals 48 damage (note I have maxed out stoneskin right now, so again that combat prediction isn't accurate), so I can easily swat him down with some poison antics.

Easy as pie, and just as I finish the job, I encounter a
second level 9 bandit. This is
incredibly good luck at this junction. After killing this bandit I'll be just shy of level 7. All the level 9's have been dealt with, so I'm really only concerned with finding the boss. If I encounter a level 8 I'll knock him out, but otherwise it's time to find the boss. I shouldn't have to explore too much more to find him...

Uhhh... this took longer than I expected, and I'm running desperately low on tiles.
On a whim I killed a level 8 golem (probably shouldn't have) and purchased the gloves of midas. I probably should have used LEMMISI to reveal the boss at this point, but it was probably too late anyways and there wouldn't be many tiles left either way. So, no regen antics for me; this is going to be a straight fight.

I completely explore the top-level, throw a few fireballs at the boss to build up some burning, and then descend into the sub-dungeon. Once you've completely explored the top-level, you're free to kill the goat and make the cultists transform. Although they become somewhat stronger, they'll be worth more XP and their most deadly abilities (teleportation and lifesteal) do nothing so long as the top-level is fully explored.
As an assassin, I only intend to be killing non-boss monsters via swift hands at this point in the game, so I really lose nothing but gain a lot of potential XP that I can reap.

I throw some fireballs, kill some monsters to level up, throw some more fireballs, and the boss's first stage is down for the count. I also convert to Mystera. I don't see myself making much use of Binlor's boons, and I haven't converted any glyphs yet (I have a total of
seven thanks to the bonus glyphs I got for being an Assassin and Binlor follower) so Mystera's refreshment boon is too good to pass up.

Stage 2 boss was fairly straightforward. I convert a glyph to make room for a tower shield, allowing me to tank through by brute force and a single mid-battle level-up. I also purchase one hit of the "magic" boon to raise my max MP to 14.
Now that I'm level 9, I can swift hands every monster except for the boss. I can also convert my glyphs (I am a goblin after all) for XP, so reaching level 10 and getting a full mana restore will be quite easy. Thanks to the refreshment boon, I get 7 MP every time I convert a glyph.

I don't even
need potions to do this. I still have most of my glyphs and a frew straggler monsters around the map. In a pinch I could muster enough resources to get a post-level-10 milestone, but I don't even need that. I finish the boss and execute the remaining monsters.

You can probably see what I meant about Bandit Smorgasbord.
This is a great example of a situation where you were able to level-up smoothly, but uncovered the boss late and needed to take a brute-force approach. I may do another run later in which I demonstrate what to do if you find the boss earlier but haven't been able to level-up so easily...