Mostly by playing farming game modes, an example is infection, as when you are a Survivor there is no score limit, thus no xp limit.
Yes, but only if it is in matchmaking.
play halo
The easiest way to level up is to feed Pokemon Rare candies.
By gaining XP
completing puzzles are the best way to level up.
There is no best way, just change monsters as you level up. edit: this guy is a dick thanks mister know it all
Train it.
By playing.. ;)
For me the best way i have found to level is by doing all the quest in each area before you move on to the next area.
Slayer is the easiest and best way. You train a multitude of combat skills as well as your Slayer level.
The best way to level up pidgeotto is to have the move quick attack and take him into diglett cave, just run around and use quick attack.
rare candy