As always, critical hits will instantly kill walkers in one hit. Focus on having a Leader with a critical bonus for the team such as Michonne 3* (+16 crit for melee teammates), Holly 3* (+12 crit for all teammates) or Andrea 3* (+12 crit for all teammates). Nitric Oxide is a good item for this roadmap as it gives +20 crit to all melee units for 3 turns.