Assuming this thing is in Ubers or its equivalent in Gen 5 and all the Dragons stay there, Kyurem will be really useful. Garchomp, Salamence, Rayquaza, even Lugia have problems with this thing, but being a slowpoke compared to all of them really doesn't help. With a Choice Scarf, it outspeeds no-items or boosts Lugia and can smash it with SE Ice moves, and Frozen World can also slow up Multiscale Lugia if you feel daring enough to run dual Ice or are prepared to Draco Meteor to hit it the turn after or get a strong hit on the switch as Lugia runs in fear. Kyurem also outspeeds Salamence with a Choice Scarf or with a Speed-boosting nature on a neutral Salamence with no boosts and severly injures or kills Salamence with Frozen World. If it doesn't and survives Salamence's attacks another hit will easily KO. Frozen World and Kyurem are really underrated. Those weaknesses are horrible, horrible, horrible, but Ice-Dragon STAB is amazing and with FB can hit Steels too.