You cant use slaves for guard/fighting, sorta mentions that in intro, even if you free them, most of them dont have level'd combat skills which takes time. Playing this for a week and half now, level 84 non cheat. If you want good fighters seduce those ones in ninja village, both unique and usefull, you can only ask them to join if your one tier below them though.
Tsunade Healer/Assassin Tier2-3, and Konan Assassin 3-4. Being assassin their crits were useful. Using stockard from areana now though as the leveling of team members is slow. There is a tab in MC's profile under friends, where if you chat with them before you dont have to wait for them to show on the map again to meet them (figured that one out a bit late).
Elements you cant really use opposing affnities, doesnt mean you cant use them. there are also hybrid spells lke
other light: light/dark, Northern Flow: wind/ice, F Dicharge: Lighting water, Flame Vortex: Fire/wind generally 15,000+ in shops
Deadly Poison i found really usefull for those enemies with a lot of health.