uhhh Theres endless ways to do this.. if i was doing it i would use switches or Death count just to be less sloppy but the easyest way to do this is to use units like this
i'll use flags as ezday stated
Example:
___________________________________________________________________
Player:Who ever is human

Conditions:
Brings atmost 0 Flags to Location (Heal - level up Counter)
Brings atlease 1 anyunit to Location (Heal - Heal)
Actions:
Modify Hit points for anyunt owned by Current Player at Location (Heal - Heal) to 20%
Preserve Trigger
(i'd do this)Comment: Heal
Ok that would work for like a level one base the next trigger would be like this if you where using kills
___________________________________________________________________
Player: Whoever
Conditions: Kills atlease 25 anyunit
Actions: Create 1 Flag at location (Heal - Level up Counter)
Then make another trigger much like the first some like this
__________________________________________________________________
Player:Whoever
Conditions:
Brings atmost 1 Flags to Location (Heal - level up Counter)
Brings atlease 1 anyunit to Location (Heal - Heal)
Actions:
Modify Hit points for anyunt owned by Current Player at (Location Heal - Heal) to 30%
Preserve Trigger
(i'd do this)Comment: Heal
as you can see the more flags you bring to Heal - Level up counter The more you will Heal and to keep making Flags just make more kill trigger but instead of 25 on the next one set it to like 50 or something

quiet Simple. Also for the Heal - Level up counter. I'd make a locations like that for each player and put them some where where ground units cant get to... NOT ON WATER... flags wont create... well thats all hope i made it easy enough for ya
