The new class in the freeware version v0.2... (spoiler?)

[Jedit]

But like...Library; it's probably the worst class in the library.

I haven't tried it yet, but if you get the Imp's blink ability that would be comedy gold.

[TigerKnee]

Imp is as empty as Meat man, actually.

[Darvin]

It's a good thing you don't get blink. That'd be an utter disaster to teleport randomly after fighting an imp. In effect, the changling has no class features in the library. The only thing you get from polymorphing to a warlock or dragonspawn is magic attack, but nothing is physical resist. You do get a few opportunities to polymorph enemies, but there's no assurances you'll get anything useful out of 3-4 shots. On the other hand, library is by far the easiest of the challenge dungeons (IMO) and I was able to take a Changeling to a pretty solid victory here.

I've also noticed that the changeling doesn't get magic attack from the wraith. Probably an oversight, and an unfortunate one since this is one of the few cases where a changeling actually gets a meaningful boost in a mirror match, bypassing the target's physical resist with its own magical attack.

[T-hawk]

Here's another trick for the Changeling in the Boss Hive. Keep a mid level goo blob around. When you want to kill a higher level monster, alternate attacks on it and the goo blob. You'll have the 75% physical resist for each hit on the higher level monster and take less damage overall. Don't kill the goo blob so you can keep it around to do again. Call it Goo-pong.

Of course it doesn't work against anything with a magical attack, and will depend on the identity of the higher level monster in other ways. But this is a hilarious way to take down Gharbad in particular and also some other slugging-type physical bosses like Frank or the meat man.

[Darvin]

That also works with animated armor and death protection. Of course, gorgons are still your best polymorph choice against a boss. Juicy, juicy death gaze.

Of course it doesn't work against anything with a magical attack

Just polymorph him then. Changeling should never be forced to fight a specific boss, it's pretty much the class's core selling point.