I don't think KPQ from lvl 21 is a good idea. Especially for warriors as they have a hard time finding partys. I suggest grinding some good exp monsters would be a better idea.
This'd be better:
Level 20-24: Congratulate yourself for getting to level 20! Now you can buy weapons and armor that you couldn't purchase before, and kill Pigs with ease (hopefully 1 hit). Now you are in for one heck of a challenge. Go to Pig Beach. If you don't know where to find Pig Beach, then follow these simple instructions:
Go to the "3-Way Split" between Lith Harbor, Kerning City, and Henesys.
If you came from the left side of the screen, go towards the middle then up one level to a large yellow flower patch.
There is a hidden teleport here, towards the middle of the patch, but a little to the right. Check to see if a teleport is there by pressing the "up" button on every small step.
Once you get teleported, you are now at Pig Beach, a place where many Pigs, and Ribbon Pigs spawn about every 15 seconds. NOTE! Bring plenty of potions with you here, for you will get insanely hurt by the constant stampede of Pigs. If you are very unlucky, you will come upon an Iron Hog here (Pig in armor). Try not to attack him. If you do though, he will follow you a lot and hurt you for about 30-60 damage. Also, at your level, you will also miss a lot more than usual on iron hogs, therefore not knocking it back and not delaying it from getting it to you. At level 21, you will be able to enter the second Party Quest which is a very good place to gain EXP, though it can take a while to get in (your damage will be low, which can make the PQ take much longer than at higher levels). Fighting Green Mushrooms at Henesys Hunting Ground II is also a decent place to train. Note: as a warrior, you will always have large amounts of hp and your weapon defense will be extremely high for your level. As such, waste less money on hp and mp pots, and buy more sniper pots/pills because this will allow you to hit higher level monsters.
You can choose to PQ from lvl 25 on. If you prefer not to do so, follow this:
Level 25-29 (solo): if you choose to go solo, now is the time to go to Ellinia Dungeon Stalks in the rooms near Ellinia. Here you will find Green Mushrooms(gives 26 exp) and Horny Mushrooms(35 exp), which is just what we are looking for. Again, bring many potions, as you will be spending much time here. Also by level 27 it might be an idea to fight wild boars in Perion. Hp loss will be pretty high because the boars are fast and they take a lot of health. Dexterity should be somewhat high to hit them constantly. Another option is to go to the Evil Eye Cave at lvl 27-28 if you've got a LOT of money to spend. If you feel like trying out Bubblings, head to the subway of Kerning City, as the first area is full of Bubblings who give decent EXP. Note: most places have "Axe Stumps", which seem to never get in these things, they give you 30 EXP and can be killed at Level 15. If you feel like burning money: go fight green and horny mushrooms at about level 20. Be prepared for a lot of damage done by the horny mushrooms, and if you have lower than 26 DEX you will miss them occasionally. If you do have less than 26 DEX, bring lots of potions and sniper pots/pills. Alternatively, go to Fish Resting Spot in Aqua Road. Lots of Cicles and Cicos there so you can Slash Blast to your heart's delight, but even with good maneuvers you'll be losing some money.
Credits go to strategywiki.