It's the same concept also !
U have to create many abilities !
Abilitie 1 >or> Upgrade 1 : (+5)
Abilitie 2 >or> Upgrade 2 : (+10)
Abilitie 3 >or> Upgrade 3 : (+15)
Abilitie 4 >or> Upgrade 4 : (+20)
Abilitie 5 >or> Upgrade 5 : (+25)
Ext......
So each time a unit dies >>>> u have to >>>> Add abilitie 1 >or> Upgrade 1 .....
a unit dies again with the same killing unit then ,,, if unit have Abilitie >or> Upgrade 1 Then Add Abilitie 2 >or> Upgrade 2 ,,, then remove Abilitie 1 >or> Upgrade 1 ...
SAME IDEA OR TRIGERING OR CONCEPT !!!!