There are several options here.
1. Take it slow, bro.
Make your team slower, yet bulkier! MegaMence runs a great bulky SubDance set, and Jolteon can easily be replaced by a slightly less speedy but stronger Electric type: Eelektross.
Eelektross@ Assault Vest
EVs (if you care about them): 252 Atk, 252 HP, 4 Def
- Knock Off
- Drain Punch
- Wild Charge
Assault Vest patches up Eelektross' mediocre Special Defence. A Brave nature cuts Speed to take advantage of TR, but Adamant can also be used. Knock Off is highly spammable and very useful, and takes a good chunk of Cresselia's health. U-Turn is to pivot, keep offensive pressure and escape threats. Drain Punch restores valuable HP. Wild Charge is STAB, and Drain Punch helps shrug off the recoil.
2. Prankster Taunt
Adjust your team to kill Trick Room before it even gets up! With the help of Taunt, you can kiss your worries goodbye. If you want a Poké already on your team to carry Taunt, go for Noivern. However, Noivern isn't doing anything specifically for your team other than Tailwind, and adds to your Stealth Rock weakness as well as your Ice and Fairy weaknesses. Consider replacing him with this Prankster user: Whimsicott.
Whimsicott@ Life Orb/Big Root
EVs: 252 SpA, 252 Spe, 4 Def
- Giga Drain
Timid for speed. Life Orb fixes Whimsicott's below average Special Attack, but Big Root promises more recovery with Giga Drain. Taunt is the crux if the set, shutting down Cress before she can move. Giga Drain and Moonblast are reliable STAB moves, one offering recovery and the other coverage. The final slot is dependant on what your team requires: Encore shuts down setup sweepers, but may be redundant with Taunt. Tailwind if you still desire extra speed. Both take advantage of Prankster.
3. Take advantage of TR
Adjust some of your team so that they benefit from Trick Room. Like I said, Eelektross has low Speed but still packs a punch, as does Bisharp without Tailwind. While not every team can be slow as a Magcargo, it helps to not have this glaring Speed weakness. Also, there is a bulkier Water type than Milotic: Slowbro.
Trait: Regenerator/Oblivious (thanks to ConixComix)
EVs: 252 HP, 232 Def, 24 SDef
- Slack Off
A physical wall set that I stole from Smogon. This tried-and-true method works like a charm. Do everything you can to get Regenerator, but if absolutely impossible then opt for Oblivious. Scald is STAB and offers the chance to burn. Slack Off is recovery. Toxic is for status and annoys the pants off of most people. Protect is awesome for scouting, temporary invincibility, Toxic stalling, Lefties stalling, or avoiding friendly fire from a teammate's Earthquake.
Feel free to take any of the suggestions I've given, or none at all.
EDIT: If you insist on Noivern, use this set:
Noivern@ Life Orb
EVs: 252 SpA, 252 Spe, 4 SDef
- Air Slash
- Tailwind/Flamethrower/Draco Meteor
- Dragon Pulse/Flamethrower/Draco Meteor
Life Orb provides more firepower. Infiltraitor allows you to hit behind Subs. Taunt is obvious. Air Slash is reliable STAB, and the one you will probably use the most. The last two slots are up to preference: Tailwind for support, Flamethrower to hit Ice-types lacking in Special Defence, and Draco Meteor for massive damage or Dragon Pulse for consistency.
If you keep Noivern, then give Leftovers to Eelektross and replace Slowbro with Tentacruel.
Tentacruel@ Black Sludge
Trait: Liquid Ooze
EVs: 252 HP, 252 SpA, 4 SDef
- Rapid Spin
- Sludge Bomb/Sludge Wave
- Protect/Toxic Spikes
Black Sludge is basically Leftovers for Poison-types, but screws over Trick users. Rapid Spin is critical, removing hazards such as Stealth Rocks (which threaten half your team). Scald is for the Burn chance, but Surf can be used since this is Doubles. Sludge Wave hits both opponents, but you can choose Sludge Bomb if you don't want to damage your partner. For the final slot, Protect protects you from friendly Earthquakes and enemy Psyshocks, or Toxic Spikes for annoyance.