Great game! And it's just huge for a one-week creation: tons of different art, several types of mobs INCLUDING BOSSES, lots of rooms and secrets. How you managed to do all of that in just one week (ALONE??? SRSLY?!?!?) is beyond me.
I loved the subtle details hinting that there might be a secret passage here - a really great feature, I'll definitely try to remember this and use it in some future projects. Loved the artwork. Loved the particle effects, all those beautiful splats, and splashes, and flying numbers. Loved the references (Isaac and the poop - that was really awesome). Loved how the brush behaves and how you have to swing it for the maximum effect. And there was this funky moment, with this boss that doesn't let you get close, which resulted in me running from him in panic, trying to find a brush upgrade, and when I finally did and killed him, ohh that was a great feeling.
Kinda the only thing lacking is maybe music - not too much of it in the first place, and when it does appear it sometimes just doesn't fit the mood. Like there's a really difficult battle going on, to the music of some kind of slow and relaxing string-piano-thingie. But again, with so much done in just a week, there are some things you just don't have the time for.
And another thing - the spikes coming out of the wall are not synced with the sound effect they make.
And that is all, everything else is AWESOME.