Devoted Combo is a green-based combo deck with no blue spells, so Mystical Dispute is only a generic 3-mana soft counter here, useful as flexible interaction against their combo pieces like Nature's Rhythm or Green Sun's Zenith when needed. Prismatic Ending is quite good against this matchup since it can exile Devoted Druid, Vizier of Remedies, or Walking Ballista for cheap, so only make this light one-card swap rather than cutting more. Your best tools are Thoughtseize to strip a combo piece before it's assembled, and Orcish Bowmasters, whose enter trigger can ping small creatures like Birds of Paradise. Keep in mind that Green Sun's Zenith and Nature's Rhythm search the library rather than draw, so they won't trigger Bowmasters. Keep Fatal Push and Solitude ready to remove Devoted Druid or Walking Ballista on sight to disrupt their mana engine.