Nope, sorry. Use this instead:
Code:
    'Adversario de nivel bajo', // txWeakerOpponent
    'Adversario de nivel medio', // txNormalOpponent
    'Adversario de nivel alto', // txStrongerOpponent
AFAIK the Spanish chess lingo uses "nivel" (level) instead of "fuerza" (strength).

Also, as you see, the order of words is slightly different. Actually "El más débil adversario" may be correct in some contexts. But "El normal adversario" isn't correct in any context though.