腦力 the way to say "mental strength" in Cantonese?
It is not good to take things out of context. Try to explain what you mean by 'mental strength', then translate that. Do you mean 'great intelligence, intellectual prowess' or 'a robustness of character which helps the person deal with any problems in an unperturbed way'

The former may be 天才, the latter you could approach from 四大皆空 or 波澜不惊。

当然以后需要翻译用粤语。