SP FARMING:

To SP farm, you want to maximize the most SP (Skill Points) you can get per monster. In order to do this, your skill masteries levels (your forces, such like Ice, Fire, Lightning, etc) must be lower than your current level.

SRO takes your highest mastery level (It's the level of one of your forces/skills, i.e. say I have Ice lv 10, and Fire lv 8, lv 10 is then my highest) and computes it against your level.

Now, let's go by example.

Say I'm level 16. I'm SP farming at the Bandit Stronghold (where if I can remember correctly, the Archers/etc are level 12/13).

Let's also say my highest mastery force is Ice, at level 10. Say then now each mob is giving me 100 EXP, and 200 SP. That's pretty good! (slow leveling but giving you lots of SP)

Now, if I use my exisiting skill points to raise my level of my Ice mastery, say to level 15 or 16, this will GREATLY increase the damage/effectiveness coming from my Ice skills, and also now, if you have seen some screenshot/personal anecdote evidence, you should be getting about then 200 EXP and 100 SP per monster.

So, in simple terms

Difference between your highest mastery skill and your level is LARGE (4-6+ levels), little EXP, lots of SP!
Difference between your highest mastery skill and your level is LTTLE/NONE, lots of EXP, little SP!

So, while you might be leveling slow, you will be gaining massive amounts of SP. Then, when you think you have enough, pump one of your skills up to your current level and you will then most likely level faster 10 fold.
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