Moves
Giga Impact is usually Slaking's most used move with its high Base Power and STAB, OHKOing foes that aren't overly bulky like Tapu Fini, Primarina, and non-Will-O-Wisp Mega Charizard X. Ice Punch is used against Pokemon like Zygarde, non-Substitute Garchomp, and non-bulky Dragonite. Hammer Arm is useful against Heatran and Mega Tyranitar. Fire Punch is mainly for matchups against 4x super effective targets that either Slaking outspeeds or can't knock it out in one turn, like Ferrothorn, Genesect, and Kartana. Earthquake is useful in matchups where Giga Impact is resisted, such as against Excadrill, offensive Mega Metagross, and Choice Band Zeraora.
Set Details
68 HP EVs in combination with 100 Special Defense EVs allows Slaking to tank attacks from Choice Specs Genecect and Choice Specs Greninja, as well as letting it take a Head Smash from Choice Band Archeops. 108 Speed EVs are particularly for outspeeding maximum Speed Breloom, as well as fast Dragonite and fast Mega Venusaur. 232 Attack EVs along with Choice Band and an Adamant nature guarantee an OHKO on fast Dragonite with Ice Punch and on Pokemon like Primarina and Tapu Fini with Giga Impact.
Alternatively, you can run 72 HP and 248 Defense EVs, with the rest of the EVs invested into Attack with an Adamant nature, to survive two +1 Waterfalls from Mega Gyarados and 2HKO in return, as well as beating non-Bulk Up Zeraora. Alternatively, you could run maximum Speed and Attack investment with a Jolly nature for outspeeding most Charizard, and always outspeeding Tapu Lele, Choice Band Haxorus and Normalium Z Porygon-Z.
Usage Tips
Remember not to send Slaking against Fake Out, Protect, and fast Substitute users such as occurring Incineroar, Aegislash, and Substitute Mega Charizard X or Landorus-T. You should use 100% accurate moves like Ice Punch and Earthquake whenever applicable instead of Giga Impact, so you don't risk the 10% chance of missing against foes like Choice Band Garchomp and Victini. When Slaking is up against stall foes that it can't beat otherwise, the optimal play is to either use Giga Impact if a critical hit can OHKO the opposing Pokemon or spam Ice Punch to attempt to freeze it.
Team Options
Bulky special Psychic-types are generally good teammates for Slaking because they cover up its weakness to Fighting-types like Mega Lucario and Pheromosa and to common heavy-hitters that rely on their abilities like Donphan, Golem, and Dragonite. Alongside this, a physical / special attacker pairing is usually a good way to cover a variety of threats. In return, Slaking can be a effective Psychic-type partner, as it can smash frail Bug- and Ghost-types like Mega Heracross and Blacephalon fairly well. While Dark-types like Mega Sableye can often be a problem for Slaking, Fairy-types like Tapu Lele, Mega Gardevoir, and Clefable generally have a good matchup against them. Teammates that cover Slaking’s ability and beat Protect and fast Substitute users include Pokemon like Volcarona, Serperior, and Greninja. Volcarona and Serperior benefit from Substitute and their Special Attack-boosting moves to pressure opposing utility Pokemon, not to mention Volcarona can hit through Substitute with Bug Buzz, while Greninja's great movepool, power, and Speed typically grant it a great matchup against most Protect and Substitute users like Jumpluff and Mega Swampert. In return, Slaking deals with bulky offensive Pokemon that can survive weaker attacks and OHKO back like non-Will-O-Wisp Mega Charizard X and Kommo-o. Sturdy users like Crustle and Magnezone often pose a threat to Slaking; naturally, Pokemon that beat them can fit well with it. These would include Pokemon like Mega Gyarados, fast Donphan, and special Dragonite. In return, Slaking covers up most faster frail foes that intimidate them like Mega Diancie and Tapu Bulu.