Conform is rather situational since Copycat hasn't been reprinted, but if you can pull it off even once, it's deadly since your opponent gets 30 extra damage per turn until they manage to retreat (in addition to 40 damage from your attacks). You can make it harder for the opponent to retreat by using Ariados MT, which increases the Retreat Cost of the Defending Pokémon, and Dugtrio PL which damages the Defending Pokémon when it tries to retreat. You could use Memory Berry to use Nuzleaf RR's attack, since it makes the opponent flip to attack no matter what, but it doesn't do enough damage.
Shiftry isn't very good in Limited since it's a Stage 2, but it could be useful if you manage to get it out.
In Unlimited, use Copycat to easily Confuse the Defending Pokémon and prevent it from attacking, and use Mirage Stadium so that the Defending Pokémon can't retreat to heal the Confusion. In effect, Shiftry completely stops the Defending Pokémon from doing anything, so the Defending Pokémon gets 70 damage per turn (30 from Confusion and 40 from Conform) and Shiftry remains at full health. Shiftry could also be combined with Chaos Gym to prevent your opponent from setting up.
Modified: 6.5/10
Limited: 4/10
Unlimited: 10/10