Slugma is a great example of a Pokemon with a movepool that doesn't match its typing. It has access to some of the best supportive options, but being a pure Fire-type with bad stats screws it up. However, with this set, Slugma can be a huge asset to any sweeper by making its setup easier. It can be hard to pull off, but if you do, it will be worth it.
Setting up Light Screen and Reflect should be your first priority, since it benefits the whole team while also making up for Slugma's rather humble defenses. Memento is the move that makes this set so deadly, as it lowers both the foe's Attack and Special Attack two stages. That means with screens up, the opponent's attacks are only doing one-fourth of the damage they would normally be doing. When you use Memento, the user automatically faints, which is why it is always recommended that you set up screens before you use Memento. Self-Destruct is also an option if you want to bring down an opponent with Slugma. While most of the time Memento is the superior choice, it is still a fine option. Be aware that Self-Destruct is illegal with Recover and Memento, though. Slugma's last move slot is completely up to personal choice. Recover gives Slugma a viable way of healing and can be used if you want to keep Slugma alive for late assistance. Yawn is a great way to force switches, and it can also put the foe to sleep. Either way, it gives you time to set up screens while the foe switches or is asleep. Flamethrower keeps Slugma from being completely shut down by Taunt and packs decent power when coming off a base 70 Special Attack, even with no investment.
The EVs and nature are designed for Slugma to get the most out of its mediocre defenses. With the given spread, Slugma reaches 23 HP, 13 Defense, and 13 Special Defense with a Careful nature, though screens make these defenses seem a lot better than they actually are. A Calm nature should be used if you are running Flamethrower. The choice between Light Clay and Oran Berry is a tough choice indeed. Light Clay allows screens to last eight turns, and this is great since Slugma probably won't be around to set them up again. Oran Berry provides a higher guarantee that you will be able to get your screens up and Memento, since Slugma can't take many hits.
The good news is that Slugma doesn't need much support from its teammates, since it will be doing the supporting and typically won't be around for long. Slugma is weak to Water-, Ground-, and Rock-type moves, so setup sweepers that resist those types are its best teammates. Mantyke and Poliwag can easily switch in on Water moves thanks to Water Absorb and can then set up Agility and Belly Drum, respectively. Croagunk can switch in safely thanks to Dry Skin and use Nasty Plot. Krabby and Totodile are also great options, since they resist Water-type moves and absolutely adore screen support. Gligar, Drifloon, and the previously mentioned Mantyke can switch in on Ground-type moves without fear, since they are completely immune. Swords Dance Snover can switch in on Ground and Water moves without fear and threaten the users with Ice Shard and Wood Hammer. Barboach and Aron can shrug off Rock-type attacks with ease and set up Dragon Dance and Rock Polish, respectively. Larvitar can also switch in and set up either Dragon Dance or Rock Polish.
While the aforementioned Pokemon are the best partners for Slugma, almost any setup sweeper works well. Dratini and Bagon greatly appreciate Slugma's support since it often allows them to get more than one Dragon Dance in. The rare, but extremely deadly Nasty Plot Houndour loves Memento support, as it allows it to set up safely, which is usually hard for it. Cacnea and Seedot can use Swords Dance, Nasty Plot, or Substitute safely behind screens. Rock Polish or Swords Dance Cranidos typically has trouble setting up, but not with Slugma's Memento support. While Belly Drum Magby may seem like a gimmick, it can be a huge threat with assistance from Slugma. The list can go on and on, but these are some of the best examples.
If you are saving Slugma for mid- or late-game, Rapid Spin support can help. Squirtle is great option as a spinner, since it resists Water. Baltoy is another great spinner, since it can easily switch in on two of Slugma's weaknesses, and it resists Stealth Rock. Pineco can use Rapid Spin as well and lures Fire-type moves, thus providing Slugma with an easy switch-in. Lastly, Sandshrew deserves some mention, since it resists Rock, and is weak to Grass. STAB Grass-type moves are some of the best moves for Slugma to switch in on since most Grass-types will switch out of it, allowing the blazing slug to set up Reflect or Light Screen. Slugma has very low Speed, which means this set can function great in Trick Room. If you are using this set in Trick Room, be sure to change the nature to Sassy and give it 0 IVs in Speed to make it as slow as possible.