L> Priest

Actually, to leech EXP you have to be either 5 levels from the attacker, as you say, or 5 levels from the monster, which is what most leeches (& boss runs) are based on. Since your shadower will be the one dealing the damage, it will get the EXP anyway, so the only question is whether the priest would get EXP as well; and the answer to that is that to get EXP, they'd have to either be level 65 to 75 (within the range of your own level, assuming 70), or 95+ (within the range of the monster's level, since himes are level 100).
Meaning in 3 levels Togain should be able to duo himes with you, technically, if her current level is 92. :')

EDIT AFTER THE COMMENT OF DrJason:
It's possible I'm wrong about the "65-75" bit and it's simply 65+ (i.e, that the 5 levels difference is relevant if you're below the attacker's level, and not above; I remembered it was both but come to think of it, I haven't properly tested such a situation).

 

Actually, come to think of it, you might be right?
I'm trying to think of a situation where I ever got to test this myself, but... I mean, if you're over 5 levels above the attacker killing the monsters, then chances are you're also within level range of the monsters themselves (that range being no more than 5 levels below), unless you're using some weird method like meso bombing himes XD
Should test this sometime... ALSO IF SOMEONE READING THIS KNOWS THE ANSWER, LET US KNOW PLEASE XD I'm curious now