When I saw this as an easy to finish 1,000 gamer score I didn't expect the headache that would come with it. For a simple game, it's not bad, actually it was fun for a while until you figured out that you would be grinding the same tactic for the next 4 hours to reach the maximum level. Around the time of figuring that out was when the headaches set in. On top of that, I encountered a bug where I lost 10 levels of progress, so if you're going for the quick completion do it in one sitting, and make sure you've got something to distract you.
The easiest way to get to level 23 (and while you progress the other achievements should just pop), pick the mode where you have to stay in the magic areas for a set amount of time. I've read that using Freeze Bombs and picking the second lowest difficulty (which will increase the time you have to wait from 30 to 45 seconds) helps you to get experience faster, but after 20 minutes or so I went back to 30 second games and not worrying too much about the Freeze bombs.
I'm probably giving more flak than the game deserves. For what it is, they have done a good job although they could have expanded the modes a little more and added a more diabolic AI via difficulty setting not just a faster mob spawn. If you want a mindless game to pass the time, then grab this one if you see it cheap, you can easily sit and play it for a few minutes at a time without an issue. Any more than 10-20 minutes and expect your hands to start clawing at your eyes.