I believe Choice Banded Tyranitar is a perfect 6th for this team.
Tyranitar @ Choice Band
Ability: Sand Stream
EVs: 100 HP / 252 Atk / 156 Spe
Adamant Nature
- Stone Edge
- Crunch
- Fire Punch
- Pursuit
This team is Lacking two things that would be impossibleto put into one role. So a decision must be made about which is the better option for the team. The first is Hazard Control, the second is A wallbreaker.
For A hazard control Pokemon, 3/6 Pokemon Resist or are unaffected by Rocks, Latios has reliable recovery, and Rotom-W can take advantage of rocks by bringing the opponents poke to lower health with pain split, so A spinner/Defogger is unnecessary on this particular team. And so the only hazard control game that needs to be played is pressuring the other teams Spinner/Defogger to keep Rocks on the opponents side of the field. One way of doing this is through a pursuit trapper, which traps the latwins, and Starmie while Lopunny and scarf Lati pressure Excadrill.
As for a wallbreaker. The poke needs to take care of The latwins, Slowbro, Celebi (Although rare, still a threat to the team) and other bulky Psychic types which trouble the team, especially Mega-Lopunny, all of which CB TTar is wonderful at trapping. It also needs to be able to hit Lopunny switch ins hard (namely Rotom-W, Skarmory, and ferrothorn.) The former can be hit with Pursuit on the switch making it harder for him to do his job later in the battle, or even harder with Stone Edge. And the latter with a predicted fire punch. A strong Wallbreaker also hurts stall with STAB boosted Stone edge hitting most major stall pokes fairly hard. Even though Clefable handles stall fairly well by herself, a little more stall hate is always welcome.
Tyranitar also gives this team a second good birdspam check along with Rotom, which threatens the Mega-Lopunny who the team is built around. And with the prominence of Birdspam I believe teams should carry 2-3 Birdspam checks.
Thirdly Tyranitar brings more a bulk to the team, which fits perfectly with the bulky offense team. With great 100/110/100 defensive stats, TTar is almost as good at taking non supereffective hits as he is at giving hits.
Finally, TTar gives this team a much needed switch in to Zard Y. Other than Clef (who is Physically Defensive), and scarf Lati (who can't switch in comfortably), this team has no good answer to Zard Y, which TTar's Sandstream ability makes him the perfect Zard Y switch in.
As for this particular set, Fire punch is chosen over superpower to Hit Skarmory harder on the switch with otherwise may give the team a small problem. The rest of the moves and EVS are standard CBTtar.
Overall, TTar covers the weaknesses that could break this team extremely well. And so It is the Last.