Hmm... There really isn't anything that is a really fast training method, but here is one of the faster ones;
Step 1: Try to train at least one unit to level 50 first. Perhaps by using self-affecting abilities that do not cost MP or by having the unit use items on itself.
Step 2: Have your other units do the same thing except they will use the items on the level 50 unit for more experience.
Hope this helps.
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