Battle Moveset

A Good Moveset for Marowak would have to be this:

Attacks:

Bonemerang
Swords Dance
Hidden Power Rock OR AncientPower OR Rock Slide
Double-Edge

Items Attached:

Thick Club

Preferred Nature:

Jolly

Strategy Using Marowak

With Jolly nature and max EV's in speed, Marowak can outrun all base 90 speed pokemon that don't have any Speed EV's.
Of course, this Marowak must have rock head, to prevent the recoil damage from Double edge. Its attack with these current EV places maxes out at 518, that's pretty impressive, and it will become even more powerful with the aid of just one swords dance. If you don't want Swords dance as one of its moves, then try replacing it with a move such as HP flying/Bug/Ghost, flying and bug both cover the grass types that Marowak is weak against (just remember that you can't have HP rock in the third moveset if you do use HP Flying/bug/ghost)
Bonemerang becomes especially useful against Pokemon who like to stall with substitute/subpunch/stall, since it breaks all subs (apart from flying types, which is immune to ground type moves).
To make up for Marowak's lack of speed, I suggest you baton pass speed from a pokemon such as Ninjask(A ninjask with substitute, which allows it to survive moves while allowing its ability to boost its speed, ready for the baton pass)

EV Corner:

Max out the EVs in Attack and Speed for Marowak with any remaining in HP

Strategy Against Marowak

Marowak is a Pokémon that you don't want to get hit by. With a decent base attack and an item that doubles that, you are dealing with a Pokémon of attack power that surpasses Groudon and Attack Deoxys (568 is Marowak's Max).
- In order to defeat Marowak, you'll need decent prediction skills and something fast with good special attack and preferably a Water, grass or Ice attack.
Prediction is required for when you switch in your Pokémon (since no one would send Marowak in versus a Pokémon with the ability to 1KO it before they can do anything), since if it takes any damage, it'll most likely be a KO or near to that. After that, you'll probably need to predict what Pokémon will be switched in to replace Marowak, since few people would leave their Marowak to be fainted.
- If you see Marowak switched in and your Pokémon isn't a flying type (or a levitator) and has a Substitute up, chances are, it'll use Bonemarang, so don't feel confident that you have a Sub to protect you.
- Also, be aware of the variety of moves Marowak has. Rock Attacks, a no-recoil Double Edge and the anti-Sub Bonemarang. Those alone guarantee Marowak to score a normal damage hit and combined with Hidden Power gives it a lot of coverage, so don't put any bets on resisting all of its attacks.
- Finally, don't try to KO Marowak by using physical attacks unless you've been doing some stat boosting. 110 Base defence is pretty good and since Marowak has no weaknesses that lie in Physical Defence, you aren't likely to get a KO. Same for using Special Attacks that are normally effective. You probably won't get the KO that you are after.

Contest Moveset

A good Contest Moveset for Marowak would have to be this for the Tough Contest best with Impish, Lax, Relaxed or Bold Nature:

Attacks:

Swords Dance
Rock Smash
Earthquake
Double-Edge

Items Attached:

Yellow Scarf

Strategy Using Marowak

If you do the attacks in this following order you should have very few problems; 1st - Swords dance, 2nd - Rock smash, 3rd - Swords dance, 4th - Rock smash, 5th - Earthquake/Double edge

Locations in Games

Ruby/Sapphire/Emerald Trade from FR/LG Colosseum Trade from FR/LG Fire Red/Leaf Green Victory Road & Seavault Canyon

Animé Appearences

Marowak has had a couple of Animé Appearences. First a trainer lost his badges in the Indigo League where he trained hard with his Marowak. After that, Luana of Kumquat Island had one in her gym battle against Ash Episode 075: Bad To The Bone! Episode 110: Pokémon Double Trouble!