"Clear any GHB on Infernal where all four heroes have a combined displayed attack of 140 or lower. All heroes must be level 40 and have a weapon equipped."
Subreddit Bonus Quest #2: The Weak Shall Defeat the Strong from FireEmblemHeroes
31 (Nanna) + 31 (Olivia) + 41 (Klein) + 35 (Olwen) = 138
A sane person would just do Michalis, or field triple dancer, or downgrade weapons and use 4-star units. Warping Klein around on Valter's map is more fun though.