There is a lot of theory in the OSR about how to support what you're calling Fiero. Winning treasure through sheer cleverness, planning and player skill is pretty core to that playstyle. For example:
- Don't balance encounters, but do telegraph them so the party knows in general terms how deadly different challenges are, and can pick between them or prepare for them. This is often called Combat as War vs Combat as Sport.
- Roll in the open. Don't fudge dice for or against the party. Be as fair and neutral as possible in your role as referee. The players should be playing and strategizing against the situation, not against you. If players die, then they die. If the big bad evil guy dies in round one because the players thought of a brilliant tactic, then bravo, they killed him.
- Present them with "OSR-style challenges". These are challenges that have no easy solution, many difficult solutions, requires no special tools, can be solved with common sense (as opposed to system knowledge or setting lore, and isn't solvable through some ability someone has on their character sheet.