Yeah, all you can do is these three easy steps:
1. Train
2. Level
3. Upgrade with skill points
For a level 25 fighter, I would try the level 20 Abyss (solo) or the level 30 Abyss (party: cleric (1 or 2), mage/archer) to level up, and the drops are fantastic. You might try more advanced Sea of Greed, and you might stop by Burning Hill with a cleric and maybe a support char. Have fun
NOTE: To be a high-def fighter, you have to go pure Endurance (END) in your stats, otherwise you'll be swamped by the other fighters, which means less party invites in KQs and absolutely no tanking nomination. To get the SP needed for long chains of skills, you have to increase your Intelligence (INT). I suggest a 1/2 END, 1/4 STR, and 1/4 INT build: good SP, good END, and fairly good Strength, which means powerful attacks.
NOTE 2: You might just spend a little dough and buy an upgrade from the Cash Shop at Outspark forums.
Good luck!