to get the best sp/exp...you have to have a 11 lvl gap between your lvl and the monster's lvl. ALSO, it's best to have a 9 gap to get the max sp you need (if your sp farming). so for example:
lvl 32(right?) ongs
your lvl : 21
your highest mastery lvl: 12
also if the average lvl of your party is below the lvl of the mob you are fighting...you'll get double sp ( i believe). for example:
you have a lvl 80 plvler, your lvl 22 char = 102(total lvls) / 2 (# of ppl in party) = 61 which is way over 32 (the lvl of ongs) so you won't get the extra bonus
but if your in a party with a lvl 80, 20, 20, 20, 20 = 160/5 = 32 = good enough for the bonus

also having only euros in your party helps a bit in exp gainage (thats what i heard anyways)