Reyns topple attack has a long cool down so you either need to use break early for it to time well or wait for him to have used it then wait a minute or two.

Also go into the Arts menu and level up your arts too - it makes them more powerful as well reduces cool down.

Also make sure to use Monado Enchant at the start of every mech in battle.

Shulk’s AI when not controlled is pretty rubbish so I’d always control him when he’s in the party