A healer (5-star siren) would be helpful, but not critical. I’d say it’s more important to work on your black and white resist dragons because:
a) In the third wave (and potentially the second), you’ll be attacked by maybe 4 powerful knights. They can really put the hurt on, even with Odin. However, a 5-star resist dragon’s skill with kill them all instantly.
b) That skill will also speed things up on the two keepers you’ll be fighting, since it does 20k damage.
c) Resist dragons are probably more critical than rainbow keepers in the Master mech dungeons anyway (it’s good to have two resist dragons as your leader and your friend’s leader, so you get 75% damage resistance of that color). The black and white dungeons are coming last, so if you work on those two dragons, you might get a crack at them.