Pert
Tkiss
Mence
Zappy
Ttar
Breloom
First of all, your team as it is has issues with Scizor, Mence, Latias, Rotom-A, and other stuff. Our job is to rework it so it can handle these threats while preparing for a DD Mence sweep. Let's look at how the existing members contribute to that:
Pert- Great, SR support and might survive to check stuff midgame
Tkiss- Meh
Zappy- Checks some stuff, but allows just as much or more to set up on it. Meh.
Ttar- If you give him Pursuit, he could help a lot with Latias and Rotom-A, since without it they're just dancing around until Ttar is gone and they have free reign.
Breloom- Absolutely nothing. Destroyed by every threat mentioned above, can only set up on slow stuff. pwns n00bs!! but can't do much against stall teams (Celebi says hi) or quick offensive teams. Replace replace replace.
Mence- Critical to the DD Mence sweep lol
As it is, I'd like to keep Pert (any lead will do, really), Ttar, and Mence. Zappy does have an important role to fill- countering Scizor. However, another poke can do that job better than Zappy and support Mence like a beast.
Magnezone @ Leftovers
Timid, 252 SpA/252 Spe/ 4 HP iirc
-Tbolt
-HP Fire/HP Ice/HP Grass
-Substitute
-Explosion/ Flash Cannon
or something like that (Scarfed could work). Magnet Pull traps Scizor. Sub up on Bullet Punch, kill with HP Fire (or Tbolt if you choose for more type coverage), and have a Substitute to deal with whatever comes your way. Works beautifully. Flash Cannon deals with random Ttars that follow, and they will. The EVs need tweaking if you use Explosion but I'm too lazy to tell you how.
Maggy has perfect type synergy with Mence. Between them, all 17 types are resisted, meaning they are each other's new best friends. Maggy also traps those pesky Steel types in Outrage's (or Dragon Claw's) way. I've tried MagneMence, it works beautifully. Do it.
You need something to deal with Mence, Latias, and random stuff. Jolly Scarfrachi w/ Iron Head/Trick/elemental punches or U-turn as filler is a beast. Also gets near perfect synergy with Mence (neither resists Psychic). Scarfrachi takes down Mences, even +1 Adamant ones. It also ***** Latias, Ttar, and anything that doesn't resist Iron Head. Serene Grace Iron Head is even stupider than Serene Grace Air Slash, with an equal 60% flinch rate coming from a hard-hitting, fast, bulky Jirachi. Scarfrachi can win you a lot of games almost singlehandedly, it's amazing.
The last slot is up to you again. The major threats we mentioned earlier are all dealt with after the new additions, so I'll just throw out a few suggestions. Latias, as always, is a great choice. Magnezone and Jirachi cover all its weaknesses. It would do a great job at luring in Steels for Magnezone to eliminate or it can eliminate them itself, either way setting up for a DD Mence sweep later. Choice Specs w/ Surf to take down Scizor and Ttar is a great choice. Draco Meteor coming from Latias' SpA with Specs even 2HKOs Scizor, taking into account the SpA drop.
Scizor is Scizor. U-turn spam with Bullet Punch as a "utility check" is always useful.
You could keep Togekiss, extra paralysis is always nice.
Your final team will end up like this,
Lead Pert
Magnezone
Scarfrachi
Scarf/ CB Tar
DD Mence
Latias/Scizor/Tkiss
a solid improvement as I see it; not nearly as weak to SR or common threats, and much more of an offensive threat on its own. Hope this rate helps!