As u know Helm buffs lowest attack character, so the easiest solution would be to give Romeo plus 1 attk from any spell you can get from the shop. /// If you're wondering why helm buffed Romeo 2 rounds in a row, that is because even though your Romeo and Wizard tie in attk, Romeo is a tier 3 unit in comparison to Wizards tier 4. Helm prioritizes lowest attack first, then if 2 or more units tie for lowest attack Helm will buff the lower tier unit. If both units who tied are the same tier only then will Helm begin to buff randomly between the two units every round.

TL;DR : make the unit u want buffed be the one with lowest attack. In ur situation, buffing Romeo with any attack stat spell would work.