It's done through code. Here's a quick and ugly way to do it:
 ifvare userdef[THISACTOR].player_skill 1 strength 50
else ifvare userdef[THISACTOR].player_skill 2 strength 75
else ifvare userdef[THISACTOR].player_skill 3 strength 100
else ifvare userdef[THISACTOR].player_skill 4 strength 125
 
Stick that in the the startup code you have for the enemy. If you get errors about userdef than do this instead:
 
getuserdef[THISACTOR].player_skill tempvar
ifvare tempvar 1 strength 50
else ifvare tempvar 2 strength 75
else ifvare tempvar 3 strength 100
else ifvare tempvar 4 strength 125
 
edit: just woke up so I got the numbers wrong 