Marowak
Move 1: Nature Power
Move 2: Substitute
Move 3: Bolt Strike/Icicle Crash
Move 4: Recover
Item: Thick Club
Ability: Prankster
Nature: Adamant (+Atk, - SAtk)
The main sweeper on one of my teams, this Marowak set is similar to the well known Prankster Groudon set with a few differences that make it unique. For one, Marowak has a considerably higher attack stat than Groudon, thanks to his signature item: Thick Club. Base 150 Attack for Groudon and nearly base 210 Attack for Marowak! This is equivalent to running Groudon with a Life Orb.
The difference in power is significant enough to do things like 2 HKO Adrian's Prankster Ferrothorn (which Marowak outspeeds), while Groudon can only shoot for the 3 HKO. Another Pokemon that Marowak can dent is Magic Bounce Giratina. Groudon tries to use Nature Power against it but comes up short:
252+ Atk Groudon Earthquake vs. 252 HP / 252 Def Giratina: 139-165 (27.57 - 32.73%) -- possible 4HKO
Even Critical Hits won't (usually) help Groudon here, as Giratina should be able to restore any damage dealt easily. Marowak on the other hand, inflicts heavier damage and works especially well if one can be fired off as Giratina switches in to take the attack:
252+ Atk Thick Club Marowak Earthquake vs. 252 HP / 252 Def Giratina: 180-213 (35.71 - 42.26%) -- 91.85% chance to 3HKO
Assuming a NP on switch in, Marowak can then keep up the offensive pressure, forcing Giratina to spam Recover. In the event of a Critical hit here, it will almost surely mean a KO on Giratina. An alternative is Icicle Crash, which can inflict more damage and possibly cause a flinch if Marowak outspeed (Base 45 gl), but the Priority isn't there.
252+ Atk Thick Club Marowak Icicle Crash vs. 252 HP / 252 Def Giratina: 204-242 (40.47 - 48.01%) -- guaranteed 3HKO
The damage isn't that different between the moves, which is why I favor Nature Power. Icicle Crash is very useful, however, against one Pokemon that stops this set with Bolt Strike: Landorus. As expected this is an easy OHKO and even if Marowak isn't behind a Sub, it's impressive 110 Base bulk has a chance to survive an Earthquake even at +2 Atk from Landorus-T.
+2 252+ Atk Landorus-T Earthquake vs. 252 HP / 252 Def Marowak: 288-339 (88.88 - 104.62%) -- 31.25% chance to OHKO
This leads me in to the alternative, the more useful against Lugia and Ho-oH Bolt Strike. We've all tried to take down Lugia with Bolt Strike from Groudon, but watched as it easily recovered the HP back with Roost and gained that Normal resistance for one turn. Well with Marowak, Bolt Strike is different, especially if Lugia just switched in at full HP with Stealth Rock up:
252+ Atk Thick Club Marowak Bolt Strike vs. 252 HP / 252 Def Lugia: 294-348 (70.67 - 83.65%) -- 68.75% chance to OHKO after Stealth Rock
Lastly I want to add that Marowak acts acts as an Anti-Imposter to some extent, due to his drastic drop in Attack without his Thick Club. Let's face it, 284 Atk doesn't do anything in Balanced Hackmons. One downside to Marowak over Groudon, it's very dangerous to switch into anything as Marowak can't take Special Attacks. U-turn-ing in, Baton Passing behind a Sub, or Revenging are the best ways to get Marowak onto the playing field. Hope this set works for you!