Yeh geoDefense + geoDefense: Swarm. There's also, Rolando 1, Sway, BeatRider Touch (though I find it quite easy) if you like a different kind of genre. Those games are really fun but challenging.
Samurai "Way of Warrior" level 3 was almost impossible, much harder than the rest of the game. Though an update was recently released that was supposed to fix the problem, not sure if it did as I haven't played the update yet.
I respectfully disagree. I find level three, is when the fun really starts in this game. I will second Eliss, and Geodefense, and will add, CS.ONE and ORBITAL.
both geoDefenses, definitely Eliss, I found it impossible when I first started but it's easy for me now space ninja (my favorite by far) can get ridiculously hard Myst (without a walkthrough )
those four would be my choices. Maybe not orbital cuz its a high score game, but definately the other 3
Level 3 was definitely the level that separated the boys from the men in Samurai (no offense to any women playing it). Alas, I proved to be a boy and had to wait for the update to beat the last stage of that level .
Metal Gear Solid Touch which is in fact very easy. AC I'm not sure what it means but I think Ace Combat which isn't out yet.
Try the F.A.S.T tutorial missions without doing it the easy way (running away). Also, Fastlane (names coincidental ) has some very hard races.