Your team looks really solid and handles a lot of the tier's prominent threats quite well. You're right- with Jellicent over Slowbro, Mamoswine is a huge problem with your team, so nice catch with that one. However, I don't think Mamoswine is the only pokemon that would give you problems- Gliscor seems as if it could hurt your sweepers a lot, especially because of it's great defnse and ability to Toxic stall Sandslash and Slowbro to death. If it has Ice Fang and or EQ, it can also dispose of Salamence and Tyranitar, and it doesn't care about Leech Seed because Poison Heal makes up for the lost damage. BoltBeam also does a number on your team, Tbolt OHKOing Slowbro and Ice Beam taking out Salamence and Sandslash.
Now, I know Salamence is part of your team's core, however I strongly suggest you switch Salamence with Life Orb Jolteon
Jolteon @ Life Orb
4 HP / 252 SpA / 252 Spe
~Hidden Power Ice
~Volt Switch / Substitute
~Thunder / Baton Pass
Your team is lacking a bit on the special side; you relied on a mixed sweeper with better Attack and a physical wall to dish out your special attacks for you. Well, with Jolteon, that's no more. The reason why I think Jolteon has more worth than Salamence here is because of it's Boltbeam coverage. Thunderbolt and Ice Beam take out most of Sandslash's counters, making it a great partner for Jolteon. Jolteon's high speed and pretty high special attack will also allow you to revenge kill a lot of threats, something that without Scizor you can not do anymore. The last two slots are optional; you can run SubPass to block status for Sandslash and allow it to have a free turn of setup. Or, you could run Volt Switch and Thunder; VS to force the opponent to make even more switches and take even more hazard damage, and Thunder to have a reliable answer to Rain.
After you implement Jolteon, there are a few minor changes I suggest you do. Tyranitar needs a Chople Berry
so it can take fighting type hits, especially from Tornadus-T. In Jirachi's Heatran set, you could try Roar
instead of HP Ice to do some phazing and weaken your opponent's team so Sandslash can sweep. I recommend you replace Slowbro's Slack off with Ice Beam
so you can defeat Gliscor.