Pokémon X and Y Walkthrough, Pokémon Move Sets: Dragalge
(Please note that the recommendations below are largely made for in-game play. A Dragalge in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. If you have alternative movesets, share 'em in the comments.)
A leafy, fragile-looking creature that's much tougher than you'd think, Dragalge is an altogether unique pokémon. It combines poison- and dragon-type moves into an effective build, one that can, with some EV work, turn into a fairly robust tank for your team.
Type: Poison / Dragon
Dragalge's evolutionary process is straightfroward, but time-consuming. All you have to do is level a Skrelp to 48. Do that and it will transform into a Dragalge.
Bug, Electric, Fighting, Fire, Grass, Poison, Water.
Dragon, Ground, Ice, Psychic.
Dragalge's stats skirt the realm of tank without quite hitting the mark. Its Defense is good and its Special Defense is excellent, coupled with a decent Attack score and a quite serviceable Special Attack. The biggest drawback here is Dragalge's rather poor HP, which, despite the high defensive scores, isn't quite enough to outlast strong moves targeting its unfortunate number of weaknesses. Its Speed is also terrible even with a good Nature, and will take a lot of effort to develop. (And probably isn't worth the effort.)
Poison Touch: Physical attacks from Dragalge have a 30% chance of incurring poison. This is a decent ability, but Dragalge is better weighted towards Special Attack than Attack.
Poison Point: Physical attacks against Dragalge have a 30% chance of incurring poison. Though neither ability is amazing, Poison Point is a little more useful than Poison Touch - though only if you aren't going for a Toxic build.
Adaptability: STAB damage is increased to double rather than 1.5x. There's absolutely no downside to this ability. Adaptability is a hidden ability.
Dragalge is best utilized as a wall, no contest. Trying to boost its Speed so it can sweep without getting hit is a waste of time; you're better off loading it down with some powerful moves which it can use to waste the competition. Special Attack is the better attack stat, so start with Sludge Wave / Sludge Bomb and Dragon Pulse for STAB damage. This is especially important if you manage to snag a Dragalge with Adaptability. Beyond that, Dragalge retains a large number of water-type moves, so Surf is always a great idea. Other possibilities include Thunderbolt, Focus Blast, and Hyper Beam. Round this set off with Rest and you've got a solid pokémon.
The alternative here is to try to poison the opposition, and your best option for that is Toxic. Let this noxious move chew up other pokémon while you set up a solid defense with Double Team, Rest, and attract, Sleep Talk, Substitute, or whatever other move might catch your fancy. (Again, if you have Adaptability, a STAB move may be in order. Dragon Pulse, at least?) Remember: You DON'T want Poison Point if your intention is to poison! Toxic's version of this status ailment is muuuuuuch better.
EV / Super Training
This depends on where you want to go with Dragalge, but I found a combination of EVs to be most effective. The most important stat to boost is HP, greatly increasing Dragalge's surviviability. Defense is next, Special Defense is a small possibility if you feel like sparing some points, and then Special Attack. Reshuffle the order a bit to give Special Attack more priority if you decide to make your Dragalge into an attacker.
Catching a Dragalge
There are two ways to get your Dragalge, though only in Pokémon Y. The first is to catch a Skrelp, which you can do on Route 8. Drop a Good Rod into the water along the beach and you should find one relatively quickly. Level it to 48 and you'll have your Dragalge. Alternatively, you can catch a Dragalge by using a Super Rod on Route 8 to reel one in. They're quite rare, so don't expect this to work immediately.
Got Pokémon X? Then neither Skrelp or Dragalge will appear in your game. Find an internet connection and use the GTS system to find someone to trade with. Your version-specific pokémon is Clauncher / Clawitzer, so I recommend posting one and asking for a Skrelp / Dragalge in return. It shouldn't take very long before somebody gives you want you want.