This guide does contain spoilers! A guide on how to gain skill exp quickly for the achievement: Stroker of Weapons' Souls
All skills consume a certain amount of SP, and depending on this amount of SP, skills will require more or less skill EXP to max at level 10.
An early game skill that consumes 10 SP will need 2000 skill EXP to max at level 10, where the highest level skills that require 80 SP will only need 500 skill EXP. This is to balance the disparity between player use of skills as lower cost skills can be spammed, will be around from the beginning of the game, while higher cost skills will be only available during late game and will be used less frequently.
Luckily for us, this means the skills take roughly the same amount of time each to max at level 10 regardless of their SP cost.
The most imporant thing to remember are that skills still gain EXP regardless if it hits an enemy or not. Which means we can take advantage of this for a nice little leveling trick:
First you'll need to warp to Earth Sanctum: End
These green crystals in the Earth Sanctum when broken, drop large amounts of SP (the points needed to use skills) which will allow us to spam skills, and accumulate skill EXP at a fast rate.
To reset the crystals, exit south to the next screen and return to repeat the process over again.
However there are some important accessories I highly recommend to get before attempting this.
There are 2 Accessories I recommend having equipped to make skill grinding go much much faster:
Training Ring II
Equipping this will raise the skill EXP you gain from using skills from 1 to 3. This ring triples EXP gain and will cut down time spent leveling skills significantly.
Equip this on the character who's skills you are wanting to level up.
Equipping this will let you gain SP from simply running around. This can help save time as its another way to get SP on top of the green crystals.
This item will not work if you don't have it equipped on one your main party members (the 3 visible characters you control)
If you want to see this method in action, I made a short video showing how its done: