if you want a creature to give alota XP try making it a better lvl through object editor.
or if you want heros to gain more XP then do this. Click on Advanced on the top menu of thw window, click on gameplay constants, then check the use custom constants, scroll down under hero XP and click on XP rating i think it is, it has a list which looks something like this
80
50
40
25
5
0
each number represents the percentage of which the hero will be getting when it kills something, delete all intergers then make a new one and make it 100 or what ever, that way no matter what lvl your hero is he will still get XP, you can also change max lvl, you can aslo make the hero get 200 XP if you want, have fun