I propose... Choice Scarf Breloom!
@ Choice Scarf
Jolly, Technician, 4 HP / 252 Atk / 252 Spd
~ Bullet Seed
~ Force Palm
~ Stone Edge
I agree with the idea that we should definitely go on the offensive with this pick, as we have Team 1's threats almost entirely covered between Heatran, Keldeo, and Celebi. What we don't really have, though, is something that can threaten any Pokemon that Team 1 selects at any time--and that's Spore.
What I love about this set is just how threatening technician-boosted Bullet Seed and Force Palm are. Force Palm, for reference, is a 90 BP move with a 30% chance of paralysis. With the combination of Celebi's Thunder Wave and Force Palm, we'll likely spread a great deal of paralysis to the other team. More importantly, Bullet Seed gives us a potent weapon that can comfortably outspeed and knock out Rotom-W. Force Palm takes care of Kyurem-B. With Spore and Choice Scarf, we can outspeed anything on the other team, including most fighting-resistant and grass-resistant threats, and put anything to sleep that isn't paralyzed with 100% accuracy. That effectively means that anything our team cannot comfortable counter, we can out-speed and shut down at the very least. This means that Breloom would act as an excellent lead, threatening sleep at any given notice--which means Landorus cannot comfortably switch-in at any point if Sleep Clause hasn't been activated. Meanwhile, Bullet Seed 2HKOs comfortably, OHKOs Rotom, and with all of Kyurem-B's mighty bulk, it's completely destroyed by Force Palm.
Stone Edge is for coverage, should Team 1 load up on flying-types. The best thing about picking Breloom now is that we can scout how Team 1 responds--do they pick a dedicated sleep absorber? If so, we can easily react and plan accordingly, and we've already forced them to adjust. Do they put a choice scarf on something even faster to force it out? If so, we've forced them to rely on high-speed Pokemon to "check" Breloom rather than counter it, which makes Celebi's Thunder Wave and Breloom's Force Palm all that much more effective, possibly crippling their response. Do they load up on fighting-type resists and grass-type resists to resist Breloom's onslaught? Then we know we can comfortable employ Stealth Rock, which will hit most of Breloom's counters for a quarter of their health every switch-in (flying-types).
In terms of synergy, Breloom may seem a bit redundant with Celebi, but keep in mind that Celebi's biggest counter--Tyranitar--can be comfortable neutralized. Breloom can switch in on anything Tyranitar throws at Celebi, barring a mixed Ice Beam or Fire Blast set, and put Team 1 at a huge disadvantage immediately. Celebi can Baton Pass to Breloom, as well, safely bringing it in against any dark-typed threats. It also doubles-up on fighting-typed STAB for Team 2, putting severe pressure on Team 1 and punishing it for choosing any steel-types. And, should this deterrent prove effective, we can easily select a Dragon-type Pokemon with our last pick and blow Team 2 wide open. It also might force Team 1 to use ghost-types or psychic-types, such as Alakazam, which we can easily respond to, as well.
Just remember--after almost any kill, we can send Breloom in and immediately regain momentum against them with either STAB or speedy Spore. Sure, we have to switch after we put something to sleep, but our team so perfectly counters Team 2 that a double-switch won't exactly do us much harm.