It aslo helps to turn off the summon boost ability, as that makes the bar last longer, summons are relatively easy to level, as long as they stay on the field, when you bar reches zero, head to the nearest save point (or door if you're in Timeless River) And you'll return with a full gauge, and thus you can repeat the procedure. I usually don't perform limits as they have a nasty habit of draining the gauge way too quickly...
When I level the gauge, I use chicken little, as he's more useful just standing around since he can cast magnet, throw bombs and sometimes stop opponents from attacking. Stitch or Peter pan are the next choices as they provide support by healing you or making the opponents drop orbs.