Despite its unique role, Ninjask struggles in RU for reasons both inherent to itself and to the tier as a whole. Ninjask's ability to passively acquire Speed boosts and Baton Pass them, along with any other secondary buffs acquired in the process, though not necessarily a simple task, immediately establishes a niche in the tier. Bug / Flying is a very unappealing typing defensively, possessing a series of weaknesses to common attacking types (as well as a dangerous Stealth Rock weakness), and 61 / 45 / 50 defenses aren't doing much to mitigate this issue. The inherently bulky nature of the metagame means that Speed boosts often aren't enough for a Pokemon to consistently sweep, meaning that Ninjask often has to either prioritize passing Swords Dance boosts (a role it doesn't perform nearly as well as that of a Speed passer) to a partner for it to apply the offensive pressure necessary to wear down the common defensive cores of the tier, or resign itself to use in the mid- to late-game in order to find a window in which its abilities become pertinent for a sweeper to effectively clean up.
Furthermore, the omnipresence of priority users, such as Entei, Kabutops, and Spiritomb, make both passing and receiving boosts a much more difficult task, forcing the Ninjask user to carefully plan their Baton Pass. Even after all that, there are still multi-hit move users and phazers that can almost completely stop any given Ninjask setup, often forcing it to reserve itself until mid-game to have any impact on the match. Ninjask is a very capable supporter of a plethora of potent Pokemon in RU, but in order to excel in this role, it requires significant support, proper prediction and judgment on the part of the user, and just a pinch of patience.
Strictly speaking, this is the only set Ninjask should be running competitively. The concept behind this set is very simple on paper: utilize Protect and Substitute to safely acquire Speed boosts, then proceed to pass them off to the appropriate sweeper. Substitute serves a secondary purpose on this set as a buffer for both Ninjask and the recipient, protecting them from becoming compromised by status or Dragon Tail and Circle Throw, while also reducing the necessity for either to take strong hits. With that in mind, it is very rare that Ninjask has either the opportunity or the necessity to use anything other than its first three moves. However, the fourth move does offer Ninjask a degree of additional utility, and does somewhat affect how it plays. In terms of sheer potency, Swords Dance is far and away the most dangerous option—while it is very rare that Ninjask finds the opportunity to both set up a Swords Dance and bring in the proper recipient safely, the results can be devastating. On the other hand, while not nearly as threatening, Toxic is a useful and self-sufficient option for Ninjask. The cumulative chip damage Toxic provides not only synergizes nicely with the cycles of Substitute and Protect Ninjask commonly undergoes but can, at times, even force the opponent to switch out to preserve the poisoned Pokemon, which can open up a free turn for Ninjask to effectively pass off its boosts to a teammate safely.
As straightforward as this process appears in theory, it requires substantially more prediction on the part of the Ninjask user in practice. While you can very well get up to +6 Speed by constantly using Protect and Substitute in succession, this not only wastes much more of Ninjask's HP than probably necessary but is also incredibly predictable and very exploitable by a knowledgeable opponent. A very simple and effective method of keeping opponents on their toes is to avoid setting up to +6 except for in the most dire situations—it might sound odd to not acquire as many boosts as possible, but once Ninjask acquires all the Speed boosts it possibly can, it's left with nothing much else to do but attempt to Baton Pass, which makes it all the easier for the opponent to nail the switch-in with a powerful attack. It is very rare that a Pokemon needs all of those Speed boosts to outpace the entirety of the opponent's team; simply boosting to the necessary level both saves PP and HP for Ninjask and makes predicting the Baton Pass all the more difficult.
Moreover, it is important to note that having a Substitute up when Ninjask attempts the Baton Pass should hold priority over almost everything else, even if this means sacrificing an extra Speed boost or two in doing so. Having a Substitute active not only completely protects the Baton Pass recipient from everything barring multi-hit attacks and certain non-attacking moves that it might have otherwise struggled to switch into, but also gives it assurance against priority users and particularly fast Choice Scarf users afterwards, which effectively mitigates any Speed boosts the sweeper might have "lost" by forgoing the extra turns of boosting with Ninjask. While it isn't necessarily easy to maintain a Substitute, by mixing up the use of Protect and Substitute, rather than simply alternating between the two, Ninjask can do its recipient a much bigger favor than any extra one or two Speed boosts ever will. However, it should be noted that without users of phazing or multi-hit moves already disposed of, these boosts are practically forfeit, as such Pokemon can quickly neutralize Ninjask or its recipient.Team Options & Additional Comments >>>
With a Pokemon as single-minded and linear as Ninjask, expecting it to do anything more than its one designated job is optimistic to say the least. Options to try to make use of its abilities in an offensive manner, such as a Choice Band or offensive Swords Dance set, could be utilized to some extent, though in the end, Ninjask makes for a very mediocre sweeper on its own. Essentially, the one other option worth considering would be more a matter of context rather than a different move or set, and that would be using Ninjask on a dedicated Baton Pass team. While Ninjask is generally better off serving as a means of supporting a powerful sweeper, it does remain the single best Speed booster for dedicated Baton Pass teams, which makes it a staple on such formats. However, due to a variety of factors, including a noticeable lack of strong Baton Pass support and teammates, this strategy is typically inadvisable under most circumstances.
Checks and Counters
As Ninjask wears itself down to the point where it can be picked off by nearly anything while fulfilling its role, Pokemon that are capable of inhibiting it from successfully passing its boosts are the best counters. The most solid, as well as the most obvious, are phazers, as Pokemon such as Steelix and Mandibuzz are capable of completely preventing Ninjask from supporting its teammates. Contact-based phazing moves from Pokemon such as Druddigon and Poliwrath fall into this category, but their low PP, imperfect accuracy, and inability to phaze through Substitute make them substantially less effective. What's more, Haze Cryogonal can similarly neutralize Ninjask's boosts while also threatening it with a STAB Ice Beam, though its susceptibility to Toxic and inability to remove Substitute without attacking makes it possible to be played around. Qwilfish, being the premier Taunt user of the tier, is capable of restricting the number of boosts Ninjask can accumulate, as well as threatening to cripple the Baton Pass recipient with Thunder Wave if Ninjask fails to acquire a Substitute beforehand. Multi-hit move users, such as Cinccino, Crustle, Piloswine, and Rhydon, are capable of circumventing Substitute and hitting both Ninjask and the recipient hard. Stealth Rock is naturally inhibiting to Ninjask, as it shaves off half of its HP upon every switch-in, restricting it to only one opportunity to set up at best. Moreover, by simply applying consistent offensive pressure to Ninjask, it is put in a very difficult position to safely pass off its boosts to a sweeper.