Reunliclus - No pokemon bar Arceus should be holding an elemental plate, because in most (good) sets only one move will benifit from it. If his ability is Magic Gaurd (Which I recommend over his other abilities) then he should be holding a Life Orb, if his ability is Overcoat, then he sould hold leftovers. Trick Room doesnt help here, as reuniclus is one of the only ones on the team that will benefit form it, and if you need to make a quick switch, or get KO'd while it is in effect, the pokemon switching in will most likely be going last for a turn or two. I suggest you replace Trick Room with Recover, as it will let you survive much longer.
I suggest you replace garchomp with this, as it will resist and kill ice types which threaten landorus, and it will counter the dark types that threaten Reuniclus and Gengar:
EVs: 252 atk, 252 spatk, 4 HP
Item: Life Orb
Vacuum Wave is STAB and priority, while Extremespeed is more priority. Bullet Punch is STAB and more priority. Close Combat is STAB and packs a punch. Since 3/4 moves on this set have priority, there is no need for speed EVs. Credit to Josh for this set.
Landorus - Soft Sand is a no-no, as it only boostearthquake, and thats just not good enough. I would strongly suggest having him hold the Choice Scarf, turning landorus into a deadly revenge killer. Keep Earthquake, Stone Edge, and U-turn, but get rid of Fly, as it is a Horrible move to be used competitively, because the opponent can safely switch to something that resists/counters the Fly user. Instaed of Fly, You sould have Hidden Power Ice if possible, and if not, go with Outrage.
Gengar - Zoom Lens on a gengar? Really? Gengar is one speedy ghost, so you will be moving first against most unboosted/non-scarfed opponents. Also, Hypnosis + Dream Eater is a really bad combo, becasue Dream Eater only works against enemies that are asleep, and hypnosis has horrfying accuracy. I use this set and it works really well.
Gengar - Black Sludge
EVs: 252 spatk, 252 spd, 4 HP
Shadow Ball is STAB, and Focus Blast is good coverage. Taunt stops foe's from setting up, and it stops most walls/stallers/clerics in their tracks. When you are certain you will be KO'd on the next move, and you can't KO the enemy first, you go down fighting with Destiny Bond. NOTE: This set is more of a suicide assassin, as you will likely be KO'd after taunting, which gives you a great oppurtunity to use Destiny Bond.
Scizor - Metal Coat is a bad item, in fact, any item that raises the power of moves of a certain type is a bad item and should not be used. Your Scizor is OK after the item is removed, but I am going to suggest a similar, but slightly better set.
Scizor - Life Orb
EVs: 252 HP, 252 atk, 4 def
Swords Dance Turns him intoan offensive monster, and Bullet Punch is main STAB and priority, and hits really hard with that technician boost. Bug Bite is secondary STAB, and hits even harder then bullet punch with technician boost, but lacks priority. Roost heals off Life Orb damage, letting him survive longer, but if you want your scizor to be more of a siucide sweeper, go with the hard hitting Superpower. NOTE: I use Roost over Superpower, and it works really well, but that is just persnal preference.
Gastrodon - The DeepSeaScale literally does nothing on any pokemon except for clamperl, so I dont eve know why you have Gastro holding it, You'll want leftovers instead. Drop Muddy Water, as itis unreliable, and highly outclassed by Scald, which should be its replacement, and replace Earthquake for Stockpile. Do that and you will have yourself one heck of a troll pokemon, one that is capable of deomlishing teams after Stockpile is up.