A pure cannon game without all the flowery stuff.
The game, while probably not an entirely original idea on NG, is quite good. Never encountered any real, repeatable glitches so far, but I have seen some strange stuff happen. Like once on level 30, shooting the top sliding platform moved the top left boulder left for some reason. But as it's not really repeatable, I can't hold it against you.
Normally, I don't like the idea of "click and hold for a meter to build up" kind of control setup. But it works well with this game, making it more of a game of skill rather than an "input angle and power" setup, which would of course be very easily repeatable.
For me, Level 6 was perhaps the hardest stage of the entire game and nearly made me put the game down for good. On my first run, I repeatedly tried bombing that upper critter several times with all sorts of angles and power settings before actually quitting the game on that first run. Seemingly deceptively simple without any gimmicks, yet so difficult because only one kind of shot can possibly work in the situation. I even thought it was where you needed to shoot a slow bomb in the air, and then shoot another bomb at fast speed after it, causing the first to ricochet and land in the upper area.
Fortunately, I persevered and decided instead to refer to your walkthrough to see how that beast was possible. That and Level 7 (reason below) were the only levels I used the walkthrough on. The rest of the experience wasn't as evil as Level 6; I still consider it way more difficult than the last levels.
Another level-specific issue is that it wasn't at first clear that I could actually shoot above the Level 7 wall. (Sure, you can say that the border up there was a little different-but the real problem is that there is even a border in the first place, which from previous experiences except for Level 4, acts as a ceiling. On my playthrough, I thought that you had to bomb the little knob on the bottom part of that wall because of the staircase shape.
Gameplay wise-the objective creatures are always the same creature, again and again. It gives that kind of repetitive feeling, really. How about adding several other types, such as one that could take more damage, or one that would only be killed by something strong like boulders?
Some others have said that powerups could be in store for the future. Sure, why not, but if you choose to implement them in a sequel, keep them on a limited special ammo basis. Some wacky ideas off the top of my head are a remote sticky bomb which could be fired on some edge and detonated at the right time to knock a boulder or ricochet another bomb for some puzzle or perhaps a super-bomb that explodes on impact and destroys wall sections.
If you could add a level editor of some sort, do so. If it's too much of an issue, don't worry too much.
Lastly, just some minor nagging regarding navigation:
There should be a way to go directly back to the level selection menu. I couldn't find one, (only options were New Game, Restart, and Walkthrough.) Why should something like that be implemented? For player convenience naturally, because some people after finishing the game might actually want to dip in and out of some stages again.
Restart and the potential Return to Menu button should be also implemented as hot keys. (probably R and ESC, respectively). Saves a mouse trip to the top
Otherwise, there's not much else to be harsh about that I can think of. Good job.