If it's a custom ability you can change the buff for each level, buf if it's a warcraft ability ?
Detect EVENT_UNIT_SPELL_EFFECT and the level of the ability, then check when the target got the buff ?
If you have a better idea ...
You cannot get the "level" of a buff (even though it's displayed in the tooltip). Your best shot is making a different buff for each level of the ability; from there you can get the level.
