Clocktower! Maybe if I played through story mode I'd know why it's called Clocktower with it's shocking lack of clocks (though there IS a tower).
The start looks a bit like this:
and then a little bit further up:
Obviously this stage's gimmick is the bomb blockl and the cracked blocks that said bombs like to create. The bomb blocks activate once stepped on and have a short-ish timer on them before they explode, causing the block to disappear and the adjacent ones to turn into cracked blocks.
Due to this, Clocktower is a very easy stage to break. Destroying every bomb block on the bottom 2 or 3 levels along with 1 or 2 pushed blocks is enough to take the entire thing down. Make sure you keep an eye on your opponent with this in mind. You don't want to lose even though your opponent put in little to no effort.
Outside of this, you need to make sure you're aware of how long blocks have until they explode. The block below you could disappear at any moment.