Spade guy activates, if by "activates" you (by which I mean me) mean you (by which I mean spade guy) put on one of the dwarves' hats and just kind of stand there on the bridge for a bit.
Hey, remember that seemingly indestructible box you (spades) had? Standing there on the bridge doing absolutely nothing for several months (by which I mean hours) gave you a lot of time to think about it and the conundrum it poses. But you only just realised that you could use it as a shield or weapon or something. If lava can't destroy it, maybe nothing can. Maybe that's the only way you can successfully defeat Hearts.