Training is the best way to level up. You can also level up in battle, and by receiving experience from the mushroom in the museum who gives free things when you give him clovers, but those methods are not as fast or reliable.
The more you upgrade your training facility, the faster you level up from training. When your training facility is at maximum level, you will go through levels insanely fast.
Completing the missions is also important. After mission 50 in each exercise, hundreds of points are awarded for each mission completed.
Strength - I consider this to be the most fun exercise and also the hardest to earn a super gold combo medal in. Since I do not grow bored of it the way I do the other exercises, I can train strength for longer periods of time. Rapid arrow key mashing is fun until your wrist says otherwise.
Block - This is my second favorite exercise. The missions are not particularly easy or hard, and the super gold combo is attainable with enough practice.
Accuracy - This is my least favorite, but at 150, it has the lowest super gold medal combo number, and it itself is an easy exercise. As long as you keep track of which target has been on the screen the longest, it is very easy.
Dodge - This is another one I like because perfect stars become easier to obtain at higher combo levels. Once you have the rhythm down, and you can press the right button at the right time, the super gold combo medal is not difficult.
Critical - I find this one almost boring because of how easy it is. Click when you see the mouse icon, do not click when you see the skull icon. If it is approached with confidence, there is no limit to how high a combo can be, other than when the player develops carpal tunnel syndrome.
@Arm_Candy what level are you? I'm at level 142, and the only real way to level up there is to clover up. A crappy dose of exp gets me 20% of the bar or so, a good dose gives 60+%. In contrast, 10-20 minutes of training earns less than 10%, but lots of clovers.
For the sake of your fingers/wrist/other computer using body part, I think shield is the simplest, assuming you have reasonable competence as the early levels are boring. The others get tiring rather quickly, and again, all you care about is clovers.
@randomblah I am at level 194, and at the time of that post, I was at level 172. All leveling up from 173-180 was from training alone, leveling up from 181-191 was from 130 clovers acquired from training, and leveling up from 192-194 was from purchasing 50k rubies worth of XP from the shop. The clovers are technically the fastest way, but it only has an effect if you have a large quantity of them, which is .
I level up every time I train for more than 5 minutes. If you are not even earning 10-20% of the bar for 10-20 minutes, something is amiss.