There are many ways, after, its a matter of what you prefer.
- Simpliest is to put a passive skill to those races at skilltrees, with decreased aptitudes. It can be pdef, pAtk, etc etc.
- A clever way is to influence formulas of skills using souls (so far, many skills are using it, no ? I don't really know shitty high chronicles :P).
- Another is to edit manually each skill (power, effect,...) It's long.
- Another is to put your idea (must be greek national idea), with 50 different configs according if enemy is an archer, dagger, blabla. Surely as long to tweak skills than doing that.