I slightly changed the set you gave, I replaced synthesis with rock slide so that Mega Sceptile can OHKO threats such as Mega Charizard Y and Volcarona with it while easing pressure off cradily to check those pokemon.Other than that the set is standard,Leaf storm and dragon pulse being stab moves on the set, focus blast is used for steel types like ferrothorn or heatran. Since the set is mixed, a naive nature is used to take hits better on the physical side.
4 Atk Sceptile-Mega Rock Slide vs. 0 HP / 0 Def Volcarona: 336-396 (108 - 127.3%) -- guaranteed OHKO
4 Atk Sceptile-Mega Rock Slide vs. 0 HP / 4 Def Charizard-Mega-Y: 288-340 (96.9 - 114.4%) -- 75% chance to OHKO
: Next up we have Rockium Z tapu bulu with swords dance which serves as a wall breaker on the team, denting pokemon such as skarmory,celesteela, Mega venusaur, toxapex etc after a swords dance under its belt:
+2 252 Atk Tapu Bulu Continental Crush (180 BP) vs. 248 HP / 252+ Def Skarmory: 222-262 (66.6 - 78.6%) -- guaranteed 2HKO
+2 252 Atk Tapu Bulu Continental Crush (180 BP) vs. 252 HP / 132+ Def Celesteela: 306-361 (76.8 - 90.7%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Tapu Bulu Continental Crush (180 BP) vs. 252 HP / 212+ Def Venusaur-Mega: 252-297 (69.2 - 81.5%) -- guaranteed 2HKO after Grassy Terrain recovery
Superpower is used for steel types like heatran or ferrothorn while horn leech is its stab move that also heals it.
: Breloom, a staple on Mono grass team is no exception here, its priority mach punch allows to pressure match ups like dark and normal and steel to some extent. Spore utility combined with focus sash is huge as it can cripple a pokemon for few turns with it and gain momentum for the team. Rock tomb allows it to take on fire and flying types such as Volcarona and Zapdos. Bullet seed is the stab move that is powered up due to technician ability, adamant nature is used for more power.
: Next up is rotom-mow the scarf user or the speed control of the team, it also provides volt switch momentum to the team. Rotom-mow with a scarf checks threats such as Scarf heracross and Mega Charizard Y with thunderbolt. Trick is used to cripple a wall such as chansey. Leaf storm hits neutral targets hard and can be used as hit and run move.
Hp ice is used for dragon types like dragonite or garchomp which can be hard to take down otherwise.
: Next we have utility ferrothorn to provide spikes support to the team which is aprreciated by offensive members on the team. Leech seed along with protect allows it to drain opposing pokemon while scouting for moves. Spikes and leech seed is a good combination to abuse, which forces switches. Gyro ball is used on the set to hit threats like kyurem-black very hard.
: Lastly we have specially defensive cradily on the team to sponge special fire hits for the team and provide stealth rock support while checking some of the biggest threats to grass with rock slide which are Mega charizard Y and Volcarona. Toxic is used to cripple walls or set up sweepers. Mirror coat can be used on the set as well to take out heatran or nidoking which otherwise excel at breaking cradily. Recover is used for sustain.
I have an alternate team that I made for a contest and it is around Mega sceptile, it is a balanced team and I have posted the link below for you:
http://pokepast.es/f7e142d33fd2fa5b