Enchant. The rest are pretty good but some have more uses than others.
If that's the case, how the hell does the reforge calculate then?
Enchant is one of the most universally useful skills in Mabinogi. It is well worth the AP; whether or not it's worth the time spent training it is another question entirely. At this point, I would say "probably not."
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Formula:
{[(Magic Attack - 2) + Mana Shield Rank Efficiency] * .004}
+ Mana Shield Rank Efficiency + Healing Wand Bonus + Mana Reduction Bonus
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IF
{[(Magic Attack - 2) + Mana Shield Rank Efficiency] * .004} + Mana Shield Rank Efficiency
IS
...greater than the strength of your Mana Shield reforge
THEN
...the Mana Shield reforge does nothing
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IF
Your Mana Shield reforge...
IS
...greater than {[(Magic Attack - 2) + Mana Shield Rank Efficiency] * .004} + Mana Shield Rank Efficiency
THEN
...the Mana Shield reforge is used in place of your magic attack and rank bonus.
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That's about how it works. I switched from Mana Shield 18 level to Crash Shot 20 level recently and the results are as follows:
Damage received w/staff and reforge = Damage received w/staff
Damage received w/out staff but with reforge < Damage received w/out staff and reforge
If you plan on using magic, the mana shield reforge is redundant.