They haven't put it very gently, but it is risky to have a team be 2/3rds weak to one type. You have two steels to counter, but they have no form of recovery, which means if they go, it's gonna be a painful end game. Stealth Rocks (on Mew or Tyranitar?) would certainly help, but you may want to consider swapping one of your psychics out if you're so willing.
Additionally, being that as you have three psychics currently- use Justified on Lucario to draw the attack boost from Dark type attacks? You could get some great momentum that way. Switch in, +1. Swords Dance as they switch out, +2. Kill everything.