I use roshi in my team, and am 100% F2P. I make sure he has a clone in front of him, bee shield, and fire main shield. I have been having good luck with him. My buddy uses earth main to shield him which helped him win some 10k power difference fights.

To me it sounds like you want a ninja that is indestructible. Roshi is probably one of the most balance ultra ninjas in the game. He has many strengths, but a low defense and resistance to make up for it. He has almost full team ignite chase, a mystery that hits everyone and destroys barriers, heals every turn, immune to debuffs, and scaling nin and defense. He isn't game breaking, but he is perfectly balanced