It was just the attacks. I recall them being random and it took me a while to work out that only the full cover black blocks shielded you from the wave. The blocks were just far apart enough to be awkward to get to if you got knocked back as well. One of my favourite tricks for that boss was using Sentries to trigger the plate!
@CLWheeljack I cycled my heroes throughout the game and almost always spent my coins upgrading the weakest characters first (followed by keeping Mario strong because he never gets switched out). I did the same with Steamworld Heist and it worked really well.
Sometimes a character might skip a tier of weapons entirely but this approach meant I could choose who I wanted without taking too much of a hit on damage output or status effects. I found it was worth doing the challenges after each world because the difficulty is just right and it allows you to grab some more upgrades before you move on. I think if I’d come back to the challenges later they’d have been too easy and I relished the difficult battles.