For each level in Attack Boost, you gain 5% of boost (for example, Claudio's raw skill gives a 15% boost, and Alex's gives a 25% boost).
From my observations concerning skills, here is - roughly - how level influences other skills:
- Poison: you deal more damage per "tick" with a higher poison level
- Knockback: enemy units get knockbacked further with a higher knockback level
- Slow: enemy units lose more speed with a higher level slow skill
- Area Attack: the range of the Area Attack - how many enemy units it affects each time - increases with a higher level of area attack.
I've not run any tests to try to crunch numbers, though, so if someone has further information to provide, I'd be happy to hear it.