This <i>can</i> fail, though I am not sure what triggered it. Leonard used Sweep the turn I had queued Surge of Power, which obviously kept him alive and made me restart the fight. The very next attempt everything worked fine.
If I may make a suggestion, if you don't need a certain ability (in this case, the dragon's non-Breath abilities) then leave that as Wildcard. That way it makes it easier for people to see subs if they don't have a particular pet.