Anyone who's played against or with Riolu should have an inkling of what it does: priority phazing. What makes it so dangerous is that if you are caught with hazards on your field, unable to halt Riolu's first Roar, and without any priority users, you are basically dead in the water against its antics of continuously forcing your Pokemon in and out of battle, which ensures your defeat will not be a quick and merciful one. That said, its low base stats, most notably Speed, hold it back from being totally unstoppable. It also requires a tremendous amount of support, with hazard setters and spinblockers being mandatory in order to be successful. The payoff can be very satisfying though if you can pull off the Roar chain with nothing being able to stop you, so potential victims shouldn't underestimate this NFE lest they risk their demise.
For those who don't know what Riolu does, here's the gist of it. Prankster negligibly boosts Roar's priority to -5, but more importantly boosts Copycat's priority to +1. What this means is that once you get Riolu into battle (preferably unscathed and free from status), use Roar the first turn. Riolu can then immediately copy its own Roar and shuffle the opponent again. Because Roar remains the only move last used, as the opponent didn't get to attack, Riolu gets to start a chain of Roars which the opponent is unable to stop unless they themselves have priority that can outspeed Riolu's (though Sucker Punch will fail, it can still disrupt the Copycat chain), so keep that in mind. However, also note that if you do anticipate an opponent's (faster!) priority attack, going for the manual Roar again allows you to keep the chain going without taking a stronger hit.
Copycat itself has uses other than copying Roar: occasionally it can be used to copy an opponent's attack, such as Sawk's or Primeape's Close Combat or Fraxure's Outrage, to finish it off should it be weakened. Be aware that Riolu's Special Attack is much too low to be able to do the same to most special attackers, unless they are clinging on with a sliver of health or have poor defenses and powerful moves. If the opponent cannot KO Riolu before its first Roar, status is the next best option to halt its efforts. A Prankster Substitute can block such attempts and allow Riolu to phaze unhindered. Protect works very well with Copycat, as it allows Riolu to be temporarily invulnerable to copy an opponent's move right back at them. The most common scenario you should be using this tactic for is when facing a Taunt user, as Riolu can block the Taunt and then copy it right back to prevent the opponent from using the move.
Substitute and Protect generally make for the best combo, but the last move can be used instead to improve Riolu's chances against certain opponents. Drain Punch gives Riolu a form of recovery and pairs well with Substitute, but its main utility is to swiftly remove most opponents that put up resistance to Riolu's Roaring chain, notably Magic Coat Bastiodon, Fake Out Kangaskhan, and Prankster Liepard. Copycat can form a good combo with Drain Punch, as it can be used to launch two consecutive blows on faster opponents, essentially dishing out a 150 Base Power STAB move that heals Riolu before its opponent's subsequent attack, an excellent weapon against frail opponents. It also gives Riolu a weapon to fight back directly against the opponent's last Pokemon, which cannot be Roared out. Toxic is even more potent than Drain Punch against that last Pokemon, though it is pretty much used exclusively for that situation. On the other hand, it does form a good combo with Protect for stalling poison damage.Team Options & Additional Comments >>>
Roar + Copycat, while potent, is pretty much all Riolu has going for it, so other playstyle options are out of the question. Focus Sash allows Riolu to phaze against much stronger threats, but is greatly inferior to Eviolite, which grants Riolu greater staying power, allows it to take priority attacks, and isn't rendered useless by entry hazards. Other movepool options include Iron Defense and Agility, which bolster Riolu's ability to withstand priority and make it much harder to outpace with priority, respectively. However, Riolu should generally try to start the chain as soon as possible, as one timely switch on its setup move and it loses a golden opportunity, not to mention neither move guards against status. Crunch is pretty much only for Natu, who is so rare it is hardly worth considering and should be handled by teammates anyway.
Checks and Counters
Riolu is easiest to stop on the turn it first uses Roar. If you can KO, status, or Taunt it, you're in the clear (though keep in mind that Taunt can backfire). However, once it gets started, stopping it becomes that much trickier. Magic Coat is the easiest way to stop Riolu, though fitting it on a team can be hard and the most popular user, Bastiodon, loses to Riolu's Drain Punch. Common misconceptions that priority easily stops it are set straight if the Riolu user manually Roars on that turn. Nonetheless, Riolu can still be brought down with repeated bashings of priority, though whether or not it's too late for your team depends on how often Roar brings in your priority user. Moves with even higher priority, such as Fake Out, ExtremeSpeed, and Protect, can stop the chain even if the user is naturally slower than Riolu, though using Protect brings the risk of giving Riolu a free Substitute, while users of Fake Out or ExtremeSpeed are either weak to Fighting and/or are just so frail that Riolu can beat them down with Drain Punch. Pokemon with certain abilities can halt Riolu's advances, such as those with Soundproof, Magic Bounce, Suction Cups, Trace, and ironically Prankster; Magic Guard and Regenerator can also prevent or counteract the damage from entry hazards Riolu's constant shuffling forces. Rapid Spin drastically reduces Riolu's effectiveness, as it needs entry hazards to fully take advantage of its priority-shuffling. At the end of the day, you simply have to outplay Riolu. Knowing when to throw out a priority move or hit Riolu with your strongest move, status, or Taunt amidst a Roaring chain is key to defeating Riolu.