Through the course of gameplay your character accumulates experience points through one of two different sources: Collecting coins and doing damage to Bullseye. Note any method of gaining money increases your XP, meaning doing special moves like riding Chickey Cantor, Grapple Kills or taunting, which doesn't involve physically picking up cash still grants you XP.
Once you've earned enough for your XP bar (located on the bottom-rightmost corner of your HUD) to fill up completely, you gain a level. With every level, your damage output, skill recovery and maximum health increases; You also gain a point to allocate into your various skills and upon reaching the levels designated by your products, their effect kicks in.
Coins provide roughly 90% to 100% of their value as XP.
Bullseye provides XP in intervals of 1.5 seconds. Through his entire lifetime of 30 seconds, assuming you're shooting him the entire time time with minimum reloads and actively avoiding coins he drops, you should be able to get about 6% of your bar filled up.
|Level||XP Required to Reach||1||-|
Note that XP gain is cumulative. Reaching level 2 will not bring you down to 0 XP. You will remain at 150 and so on.