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I noticed that Silva can also level up her Monster Summon Skill and gain Skill EXP from it. Apparently by summoning, obviously. She was able to gain 10 Skill EXP with each Barbarian Monkey, which were 14, and she gained 40 Skill EXP from the Armored Rhino. Based in this calculation, it could be said that she gains 10 EXP every 100 EXP.
This means that she get 1 Skill EXP for every 10 MP. Interesting. I guess it would be better to make her summon a lot of Rhinos and Monkeys equally. I guess there's not much inventory or storage limitations aside from the dungeon space itself.