Or, we could have a system akin to The World Ends With You where you manually reduce your level (only at the magia plate, of course) to increase the chance and number of rewards.
It would not only help keeping a decent income of cystals when helping low level friends, but also allow some people to try challenging some NM at a reduced level. It would increase the end reward, at the risk of reducing one's contribution or, if too greedy, being too weak to survive some stray AoEs.