9 gap I believe gives most SP, past that point I *think* XP doesn't drop, nor SP go up.
full share pt is best, put some lower level chars in there to lower the party average below the level of the mob.
Delevel them when they get like level 26/27 if I remember correctly.
and Tasdik beat me to it, good, my info is confirmed

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iSRO - AsparTame 8x - retired
Salvation - Aspar 70