fusing commons into uncommons before using them as xp for example? (not sure if this is cost effective in this game)
so far all i have in using the same element for a slight boost.
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For an extra boost if you train the same fighter into itself you get a bonus ON TOP of the elemental bonus. So making little bundles by piling the same fighter into itself is pretty effeciant and saves on coins. I've found that 7 commons of a matching element being put into and UC or 3UC into an Identical UC are pretty optimal.
I'm pretty sure that levelling fusion fighters before putting them into another one won't help a lot. If it does, it's very minor. I put my Derro (about level 20) into one of my other fighters and it went up the same or maybe a little more than usual.