Thanks to its high Base Power attacks, just good enough Speed, and Rock Head, Relicanth can threaten to bulldoze through any team with correct prediction. Head Smash offers Relicanth the ability to obliterate most Pokemon while causing huge dents on even those that resist it. Aqua Tail goes hand in hand with Head Smash by smashing most Ground- and Rock-type Pokemon, while also hitting Fighting- and Steel-types for neutral damage. Waterfall can be used instead of Aqua Tail if you dislike imperfect accuracy, though at the cost of lower damage. Earthquake covers every other Pokemon that resist, take little damage from, or are immune to Head Smash and Aqua Tail - Toxicroak, Quagsire, Poliwrath, and Registeel are a few examples. The last slot is mainly a filler, but Double-Edge gives Relicanth a wide range of neutral coverage should you find a situation where you can't trust your prediction skills.
To demonstrate the power of this set, here are some damage calculations (with a Jolly nature):
Head Smash vs. 252/252+ Uxie: 48.3% - 57.1%
Head Smash vs. 252/0 Donphan: 31.8% - 37.5%
Head Smash vs. 252/252+ Milotic: 60.6% - 71.8%
Head Smash vs. 0/4 Venusaur: 110% - 129.9%
Head Smash vs. 252/252+ Slowbro: 49% - 57.9%
Aqua Tail vs. 252/0 Donphan: 76.6% - 90.6%
Aqua Tail (-1) vs. 252/252+ Hitmontop: 28% - 33.6%
Aqua Tail vs. 252/4 Registeel: 33% - 39%
Aqua Tail vs. 0/4 Torterra: 48.9% - 58%
Waterfall vs. 252/0 Donphan: 67.7% - 80.2%
Waterfall (-1) vs. Hitmontop: 25% - 29.9%
Earthquake vs. 252/4 Registeel: 49.5% - 58.2%
Earthquake vs. 252/4 Poliwrath: 34.4% - 40.6%
Double-Edge vs. 252/252+ Quagsire: 31% - 36.5%
Double-Edge vs. 252/4 Poliwrath: 41.1% - 48.7%
With a Jolly nature, Relicanth reaches 229 Speed, outspeeding most walls, mainly defensive Leafeon, defensive Arcanine, and support Uxie. Moreover, it allows Relicanth to speed tie with positive-natured Exeggutor, Ursaring, and Omastar. However, using an Adamant nature still puts Relicanth in a useful Speed tier with a stat of 209, as it lets the rock fish outspeed Milotic, Blastoise, and neutral-natured max Speed base 50s while increasing Relicanth's offensive prowess. The EVs are self-explanatory: max Attack to deal the highest amount of damage possible, whereas max Speed lets Relicanth outspeed as much as it can. In the last slot, Stone Edge or Rock Slide can be used in case Head Smash runs out of PP. Rock Slide gives Relicanth a reliable Rock-type attack, though Stone Edge offers more power. Choice Scarf is a valuable alternative item on this set, as with an Adamant nature, Relicanth attains 313 Speed, or 343 Speed with Jolly. However, Relicanth becomes quite weak offensively, but it can still be strong as long as Head Smash hits. Another option for this set is to use Swift Swim as the ability. In the rain, Relicanth will become fast enough to outspeed the entire unboosted metagame, and with a Choice Band intact, it will present a formidable threat to the foe.
Relicanth has trouble against bulky Fighting-types, such as Intimidate Hitmontop and Hariyama, and bulky Grass-types, such as Tangrowth, Exeggutor, and Leafeon. As such, having a Flying-type such as Moltres and Scyther can be extremely useful for switching in on these threats and eliminating them with Air Slash and Aerial Ace, respectively. Faster Pokemon are also nuisances to this set. Paralysis support provided by Registeel, Uxie, and Clefable can cripple such threats. Lastly, Relicanth appreciates type synergy from the likes of Venusaur, Rotom, and Leafeon. Venusaur has the ability to sleep a foe, Rotom can spread burns and annoy teams with a Substitute set, and Leafeon can provide Wish support and phazing.