Thanks to Quiver Dance, Mothim can actually be a very threatening sweeper, as the combination of his great Special Attack, usable Speed, and Tinted Lens allow him to dish out some serious damage. Because of Tinted Lens, Mothim actually has perfect neutral coverage with just his STAB moves, which makes him incredibly difficult to wall. Mothim's amazing coverage also separates him from Lilligant, who is forced to utilize a mediocre Hidden Power type for coverage, and, no matter what type she chooses, she is still going to be walled by certain Pokemon. After a Quiver Dance, Mothim is extremely powerful and respectably fast, outspeeding everything up to maximum Speed Swellow, making him hard to revenge kill, and he can 2HKO most Pokemon bar bulky special walls. Though he needs support to deal with those troublesome Pokemon, once they have been removed, Mothim will easily rip through the opposing team.
Because Bug Buzz and Air Slash work so well together, Substitute is recommended in the last slot in order to protect Mothim from troublesome status attacks, ease prediction and allow Mothim to do some big damage, without having to use Quiver Dance first. Substitute allows him to scout what the opponent switches in, and allows him to choose between Quiver Dance, Air Slash, and Bug Buzz depending on the Pokemon the opponent switched in. For example, if they switch in something that Mothim can outspeed and KO after a Quiver Dance, you should use Quiver Dance as it breaks Mothim's Substitute and proceed to KO it. If, however, they switch in something like Entei, who would be troublesome, Substitute allows you to attack the troublesome Pokemon with a Tinted Lens boosted attack as they break Mothim's Substitute, and then switch out. This can sometimes allow Mothim to wear down the opposition by himself. However, if you feel that Mothim would best be used as a late game sweeper, feel free to use Hidden Power Ground to nail foes like Magneton and Probopass for some nice damage.
With the given EVs, Mothim outspeeds everything up to maximum Speed Swellow, while hitting as hard as possible. A Modest nature is a possible alternative if you wish to have a huge amount of power on your hands, but Mothim usually needs the Speed from a Timid nature since he will fail to outspeed many prominent threats, such as Archeops, Scolipede, and Sceptile. However, a Modest nature makes Mothim incredibly powerful and ensures that Mothim will 2HKO certain Pokemon. For example, with a Modest nature and a Life Orb, he 2HKOes specially defensive Flareon with Hidden Power Ground. In terms of alternate coverage moves, Hidden Power Rock can be used instead of Hidden Power Ground if Moltres ends up being a huge threat to your team, as Mothim's +1 Air Slash cannot OHKO without Stealth Rock. If Substitute is used, Leftovers are the recommended item, as with a Life Orb, Mothim will faint far too quickly, but if a coverage move is used instead in the last slot, Life Orb becomes the superior option to allow Mothim to dish out as much damage as possible before fainting.
Rapid Spin support is a necessity when using Mothim, as the 50% he loses from Stealth Rock can decide whether or not he will be able to successfully sweep. Cryogonal, Hitmonchan, and Kabutops are wonderful choices, as the former two are very reliable at spinning away hazards, while the latter can also set up Stealth Rock to aid Mothim's sweep. The other entry hazards in Spikes and Toxic Spikes are also great in aiding Mothim, and these can be provided by the likes of Omastar and Roselia. However, even with full entry hazard support, Mothim cannot muscle past special walls such as Clefable and Cryogonal, so using a powerful physical attacker such as Entei, Medicham, Aggron, or the aforementioned Hitmonchan is recommended to remove these troublesome walls and allow Mothim to sweep.