For monster classes each level you breach is x100 for EACH Level above you while it is divided by ten for each level below. For creatures more evolved than you slaying them depends on hidden statistics like rarity of race or stat advantage it takes a lot to calculate this so please be patient with me.
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Killing through secondary methods like Poison (Not produced by you), Falls/Gravity (That wasn't your magic), Fire (That was caused by you but isn't fueled by your magic), Drowning/Suffocating them (You have to psychically/Magically be causing it), etc. You will get 1% of the exp they would have provided but if you are in a team all get 100% exp they would have (Yes this is abusable Easily) but they still have to inflict at least some kind of damage to count. You can also "Steal" exp by wounding an opponent that someone is chasing if they kill them you will still get 100% exp.
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Killing through tertiary method will result in no EXP.
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EXP is the life energy of a lifeform the reason it can be given out at 100% more then 50 times is due to the world constantly propagating life energy to spawn more creatures this is why once creatures evolve suddenly there will be more of that creature.