Really easy - but that isn't a bad thing, there is no grinding to artificially inflate the game length. I'm just really glad they didn't have a 'get all in-game achievements' so I didn't have to listen to the stupid dialogue and whiny voices.
Every level can be easily beaten without grinding - I beat the game in a single play through, the only level I lost was when my phone rang. The only level I replayed was level 10 once to kill the Toaster (need very close to max damage upgrades to kill it in time, but don't need them fully maxed).
In general, put a single point in all skills at the start, then just all points into damage/attack speed/critical damage upgrades (and bullet number as soon as you are able, of course). Throw a couple hundred into health and speed upgrades around level 5 or 6 but you don't need more than 3ish levels in them to easily beat the game (I beat every level with 3 points in health and move speed until I had maxed every damage skill).
The skills are very meh. I didn't put a single point in until I had maxed out all the damage skills, didn't need them (beat the game with a single point in all skills). The bubble only applies if you are getting hit, wolf does laughable damage even at max level, and while the line shock is good, a single point is enough to clean up any enemies you aren't already killing with regular shots, none of the regular enemies last more than a second as long as you keep your damage upgrades up anyway.
I broke one of my main rules again with this game a while back -- "Don't play games until they get badges." (I've been burned too many times by having to replay a game to get some badges.)
Anyway, can't remember why I broke my rule, but it worked all right for me -- I opened up the game and all the badges registered for me right away.
Follow Olinser's advice and you should be good to go, if you haven't played it already.
6/6 in less than a hour without any problems. The game reminds me of Sky Quest, but IMO is far better (yes, mainly because of the length).
Also, all achievements w/o problems.
Haha! I felt dizzy with the background moving, but this is totaly awsome haha! I wish there's an achievement that you killed all the monsters in that stage with complete lives haha!