A little easy?
Is an understatement... it's WAY TOO EASY!!!
I glitched on my first try with level 10... my second run had no problems, except that I had nothing to spend my money on. To fix the glitch of being stuck after killing/destroying everything, would be to create a reset to reattempt the level.
Once you've achieved level 4 or 5, you should have enough money saved to purchase the best weapon and 'grape' grenades... buying anything else is a waste. After which, however, you'll have nothing to buy for about 15 of the 20 total levels. Make the map bigger, create ridges that I would have to jump up to to destroy a tunnel, create more weapons that do not have explosions, make the best weapon cost A LOT more, etc.
The resounding blast from the $3000 weapon literally vaporizes every enemy with little effort. If you merely click and hold with the gun aimed at the ground, you'll create an impenetrable barrier that will kill anything that touches it (except you, of course)... this, by itself, makes the game so easy... it's disappointing after a few levels with it have passed.
The worst part was that you can repurchase weapons/grenades when you click on them... meaning, after you already OWN the weapon, clicking on any other item or even the ones you've already will force you to use it. After I already owned the best weapon in the game, I decided the buy the other stuff to see what the blast looked like... well, I was definitely surprised to find that I couldn't switch weapons... I had to BUY again the weapon that says OWNED under it! >.< ... this really needs fixing, it's crap to downgrade weapons and be forced to buy an item you already own. D=<
YOU WIN! YOU HAVE COMPLETED ALL 20 WAVES! TRY AGAIN