When considering Toxicroak for a position on an OU team, it is critical that his unique assets over his more popular comrade, Lucario, be the dealmaker. Toxicroak is immune to the combination of Toxic and Water-type attacks, which allows him to switch in and set up against many of the typical bulky Water-type Pokemon in OU, something that Lucario cannot do. This ability to set up on the omnipresent Water-type Pokemon in OU is almost single-handedly responsible for Toxicroak's viability in the tier.
Despite that quality, Toxicroak has issues with his middling Attack and Speed stats. Lucario has a higher Attack, higher base Speed, an arguably superior priority move in ExtremeSpeed, and more opportunities to set up with his superior defensive typing and multiple 4x resistances. Despite Toxicroak's handy resistances to Dark- and Fighting-type attacks, his weak defensive stats prevent him from even avoiding 2HKOs from popular users of those moves. That Lucario exists is crippling to Toxicroak's presence in OU, but with Toxicroak's unique immunity and resistance palette, he can still be a very viable and powerful threat when given proper support.
Every fast and powerful Fighting-type Pokemon's dream is to be the next Lucario, and this is Toxicroak's bid at that role. While Lucario boasts higher Attack, higher Speed, and better priority, Toxicroak can do what Lucario cannot: set up on the common bulky Water-type Pokemon of OU. Swords Dance Toxicroak hits hard after a boost, and many Pokemon are KOed by his diverse type coverage and powerful priority.
Swords Dance is the crux of the set and what makes it all possible. Unfortunately, Toxicroak lacks the quintessential Close Combat that Fighting-type sweepers love, so he must instead rely on the inferior Cross Chop or Low Kick. Low Kick is more reliable than Cross Chop, especially with its better accuracy, but has serious trouble damaging light Pokemon such as Jirachi and Vaporeon. Cross Chop hits hard against everything, but one miss will likely spell doom for the already-troubled Toxicroak. Sucker Punch is the priority move of choice here for its ability to OHKO Gengar, Starmie, and other Pokemon faster than Toxicroak after a Swords Dance boost. Throughout the analysis, you'll notice that Sucker Punch is referred to as inferior to Lucario's ExtremeSpeed, and for good reason. ExtremeSpeed always succeeds, whereas a SubSplit Gengar can use Substitute to defeat a Toxicroak that uses Sucker Punch. As such, prediction is key to the success of this set. The last slot is open to Stone Edge, Taunt, or ThunderPunch. Stone Edge allows Toxicroak to beat the many Flying-type Pokemon that threaten him, such as Dragonite, Zapdos, and especially Gyarados. ThunderPunch is an option for having a reliable and consistent move to use against Gyarados and Tentacruel without having to worry about Stone Edge's 80% accuracy. Taunt enables Toxicroak to beat Pokemon like Rotom-A that might otherwise attempt to set up on it. Also, despite setting up on Water-type Pokemon, Vaporeon, Milotic, and Suicune can easily outstall this set because of ThunderPunch's low Base Power. For this reason, Taunt is valuable to the set's success, despite the reduced coverage.Team Options & Additional Comments >>>
It was hard enough finding the options such that Toxicroak could even be viable in OU, so there's really not much to be said here. Nasty Plot does deserve a mention for boosting Toxicroak's Special Attack stat and enabling the use of Vacuum Wave, but that type of set would be just as vulnerable to Gyarados and other typical Toxicroak responses, while adding more weaknesses in the form of Blissey and Snorlax.
Checks and Counters
It's pretty redundant to just list Lucario's counters here, but there isn't much else to be said. Bar bulky Water-type Pokemon, anything that beats one of Lucario's physical sets also beats the similar Toxicroak set. Hippowdon, Gliscor, and Swampert all put Toxicroak in a world of hurt with STAB Earthquake and great physical bulk. Because of Toxicroak's lack of a reliable physical Dark-type move, bulky Pokemon that the Dark-type covers will bother Toxicroak immensely. Rotom-A and Dusknoir can both switch into Toxicroak and burn him with Will-O-Wisp, while Cresselia walls him to kingdom come if it doesn't attack. Even Gengar, a Pokemon that Sucker Punch really wants to hit, will beat Toxicroak if Gengar carries Substitute. Choice Scarf Tyranitar, while not necessarily a safe switch-in, can soundly beat Toxicroak if carrying Earthquake.