Cherrim has the perfect ability and movepool to be an effective Sunny Day supporter. While Aromatherapy is great for removing status from the whole team, Grass Whistle can put opponents to sleep and allow Cherrim to set up for free if it hits. The third slot is reserved for a STAB attack, and Cherrim is not short of options to choose from. The STAB move of choice is Energy Ball, providing reliable Base Power and perfect accuracy. Another move you may consider is Solar Beam, which has 180 Base Power when taking STAB into account. Solar Beam's detrimental side effect is circumvented through the use of Sunny Day, so its usability is skyrocketed. Grass Knot is another a good option, as it will deal more damage than Energy Ball to heavier opponents like Milotic, Slowking, and Rhyperior, but unlike Solar Beam, is effective outside sunlight. The final moveslot is for type coverage; Hidden Power Fire allows Cherrim to hit other Grass-types with a 105 Base Power attack in sunlight, while Hidden Power Ground deals with Fire-types like Houndoom and Arcanine. Both attacks also hit Registeel super effectively. Cherrim also gets Weather Ball, which becomes a 150 Base Power Fire-type attack in the sunlight, though it's rather useless outside of it, as it'll be a Normal-type attack with a measly 50 Base Power. Heat Rock is the ideal item for this set as it prolongs the sunlight, and it is recommended you do not swap it for anything else.
The EVs on this set focus on Cherrim's special stats; maximum Special Attack gives Cherrim some power behind its attacks, while maximum Special Defense with Flower Gift gives Cherrim 381 Special Defense in the sun, making it much bulkier on the special side. Alternatively, you could use a Calm nature with a spread of 252 HP / 4 SpA / 252 SpD for maximum special bulk. You could also focus on Cherrim's lower physical Defense by giving it a Bold nature with 252 HP / 252 Def / 4 SpA. In either of these cases, you should consider Leech Seed or Synthesis on the set as Cherrim will be aiming to stick around for much longer.
Houndoom is an excellent partner for Cherrim, as it can switch into Fire-type moves to gain a boost from its Flash Fire ability. Houndoom also reduces the necessity for Hidden Power Fire on Cherrim, as it can defeat opposing Grass-types with its own STAB Fire-type attacks. Registeel is also a good partner, as it can switch into Flying-type moves with ease. This is helpful against the likes of Swellow, Scyther, and even Drapion. Bulky Water-types, such as Milotic and Slowking, are effective against both Fire- and Flying-types, especially Moltres. Unfortunately, their STAB moves are weakened by the sunlight, which is never a good thing.
Pokemon that can take advantage of the sunlight make obvious partners for Cherrim. Fire-types like Moltres, Houndoom, and Arcanine can enjoy their STAB moves boosted by 50%, and refined access to Solar Beam, which has no charging turn when used in sunlight. Some Grass-types also enjoy the sunlight due to their abilities; Chlorophyll Pokemon such as Tangrowth, Exeggutor, Shiftry, and Victreebel would greatly benefit from the Speed boost, while Pokemon with the Leaf Guard ability like Leafeon and Tangrowth will appreciate the status immunity. If you decide to use Hidden Power Fire on Cherrim, a way to deal with opposing Fire-types is very important to your team's success. Bulky Water-types like Milotic and Slowking are excellent choices, as they have great HP stats and above average defenses, along with STAB Surf and decent Special Attack with which to deal with them. Dugtrio is another great choice as it can trap the opponent with its ability, Arena Trap, and proceed to KO the grounded ones with Earthquake. Regirock and Rhyperior are two other options as they can switch into Fire-types with relative ease and either set up entry hazards or Rock Polish, respectively. Alternatively,they can pummel the opponent with STAB Stone Edge. If Cherrim is unable to set up Sunny Day, another Pokemon should carry the move just in case. Defensive Pokemon like Uxie and Mesprit make good choices as they can further support the team with screens, Stealth Rock, and paralysis. You may also choose a more offensive, suicidal approach to this. Exeggutor can set up Sunny Day, then sacrifice itself with Explosion, while Ambipom can set up weather and escape with U-turn.
