Backtracked through Lava Pits and completed everything there so now I'm left with only the ultimate challenges. They weren't particularly hard, some like 'No Pain, No Gain' required distributing power orbs to boost Luigi's team jump range and some trial and error before I beat it. I used Mario's Hero Sight (with obviously weaker blasters) to draw the smasher at the spawn close enough to push him into the zone and used Mario to throw Luigi in position to be bounced. Not sure what the "intended" solution is but it worked. The secret stage was baffling easy, I was well equipped but still.

I planned to finished more of the ultimate challenges today but I had to repeat UC-3 because the game crashed; worst of all on the winning turn. Managed to clear it though, albeit not as cleanly as the first attempt. Haven't found where UC-1 is yet, I presumed near the Ancient Gardens but I couldn't see any switches to activate.