Ironclad 1 - defeat vs. The Collector (Act 2 boss):
My damage engine was too slow. I was godlike once I got everything rolling (Vajra relic, Demon Form+, Inflame+, Inflame), but it took too long to set up, and my deck was lacking in defense. I ended up having to rest at nearly every campsite in the latter half of Act 2 and then got splatted by the boss.
Afterwards, HGR said I should've taken Corruption as my Act 1 boss reward instead of Demon Form.
(1) Demon Form is "an unreliable win condition" because it can be at the bottom of your deck and screw you over. He called it a newbie trap.
(2) Corruption would've synergized great with my deck, since I got Venom Egg early (all skills come pre-upgraded) and also had Feel No Pain.