I happen to love this prickly little guy, and even though they don't have much to work with, I came up with two movesets that, in a perfect world, could work!
Maractus @ Sitrus Berry
Adamant
Water Absorb
• Cotton Guard - Amazing move that'll boost it's defense up and make decently tanky!
• Seed Bomb - Good STAB move.
• Drain Punch - On top of Water Absorb, you can use Drain Punch to heal Maractus up!
• Knock Off - My favorite move; can eliminate pesky items and do good damage at the same time!
+252 HP +252 Atk +4 Def
This guy isn't gonna get anywhere with Special Defense, so being physically tanky with good HP and Defense, along with plenty of investment in physical Attack. The Situs Berry can help sustain you while setting up. Personally, I prefer this set out of the two! This moveset requires ORAS move tutors.
Maractus @ Absorb Bulb
Modest
Storm Drain
• Spiky Shield - Stalls a turn and can damage attacker upon contact!
• Cotton Guard - Amazing move that'll boost it's defense up and make decently tanky!
• Accupressure - Boosts random stat for self or ally.
• Giga Drain - STAB move that heals up Maractus.
+252 HP + 252 Sp. Atk +4 Def
Since Storm Drain will raise Maractus's Special Attack, it can bear without Water Absorb and makes up for it with Giga Drain. It's Special movepool is extremely shallow, so I had to make do with status moves. I'd say to run this Maractus next to a wall (or just Specially tanky) Pokémon with Surf. The Absorb Bulb would work well on this Pokémon, hence me choosing it. You could also have it run Toxic and Protect to help stall. I recommend Mantine.
I haven't tried out either of these sets, but tell me if they work out! - Artsy