Lets say I want a defensive, but hard hitting Swampert.
I max its Attack stat, but what do I focus on next for its bulk?
Hp or Defence? And why, if possible.
Depends several factors:
All Options or Kalos Only?
Utility or Assault?
Singles or Doubles?
Lv 50 or Lv 100?
We will give you only a few offensive tank options but with extended commentary depending on your needs.
Utility Tank - Atk and HP
Classic Swampert is probably the most straightforward set; explains why everyone will suggest you this one. Still, depending on the metagame you may still want to make a few tweaks.
Earthquake and Waterfall are staples of every Swampert. Kalos-born ones do not have access to Ice Punch and Superpower, so their replacement is Ice Beam and Hammer Arm respectively - Avalanche may work but is a bit unreliable and may be a bit weak. So if you only have access to Kalos and do not plan to include Avalanche, consider a Brave nature over an Adamant one for a stonger Ice Beam. In Singles, non-Kalos breeds can set up Stealth Rocks; their cousins in Kalos have to stick with Roar or Toxic. In Doubles and Triple, the Utility can and should use the powerful but poorly distributed Wide Guard, while Rock Slide and Protect becomes an option in the coverage slot.
As for the EVs, it needs mostly HP and Atk. In Singles, you do not want to give Specs-Latios a free kill. At LV 100, a 240 HP / 248 Atk / 20 SDef will never be OHKOed by Latios, providing you haven't taken prior damage in any form. At LV 50 it looks like 208 HP / 236 Atk / 68 SDef. Remember that staying in Latios is unwise unless you are sure you can take it out. Avalanche is a straight KO while Ice Beam doesn't even come close to a 2HKO. The only other threat we can think off where a 2HKO can be prevented is CB-Azumarrils Play Rough. But in order to survive two hits from it you have to compromise so much power that you can't even 2HKO it anymore - and that's when ignoring Latios. Checkmate!
Fortunately, there there aren't many Specs-Meteor in Doubles, so that is something you don't need to worry about too much. As said, one of Swamperts biggest perks is Wide Guard. Mr. Mime is another X-Guard user with the same base speed as Swampert and comes with Fake Out. Because both moves have the same priority, you will need a bit speed if you are concerned with Mr. Mime and don't want to rely on a dice roll. Like 240 HP / 252 Atk / 16 Spe with LV 100, or 236 HP / 252 Atk / 20 Spe with LV 50. Use a Sitrus Berry or a Rindo Berry instead of Leftovers. Be sure to include Rock Slide or Stone Edge when using the Rindo Berry as it allows you to survive Charizard Y's Solar Beam and to retaliate.
Assault Tank
With the addition of Assault Vest in this generation Swampert can take a fair amount of punishment before going down. Its support options are now limited and it is even more dependant on where it comes from. Earthquake is a staple as always but you might want to include Scald instead of Waterfall if you are feeling lucky, especially if it is a Swamperts from Kalos who needs to fall back at Ice Beam anyway. Unfortunately, Ice Beam is the most effective against physical monsters like Landorus-T, Salamence (MixMence is actually rare in Singles) and Gliscor while that Assault Vest Swampert is supposed to confront more special attackers. Hammer Arm is a rather meh option against Ferrothorn but its only answer.
With minimal investment you can survie two consecutive Draco Meteors form Specs-Latios and return with a powerful Ice Punch: 240 HP / 244 Atk / 24 SDef with LV 100, or 220 HP / 228 Atk / 60 SDef with LV 50. Do not forget that the advantage is nill if you are running Ice Beam instead of Ice Punch or if you switch into it. You can safely switch into Mega-Ampharos' Dragon Pulse without being 2HKOed, though, which is nice considering Ampharos is so bulky that is survives an Earthquake otherwise.
We don't recommend AV Swampert for Doubles, generally but especially when the Item Clause is active. Not having access to Wide Guard kills its niche and the AV is much more useful on Pokemon who can provide a form of passive or active support like Intimidate or Fake Out - or both, in the case of Scrafty.
People! I need a good moveset for Mew, any suggestions? I dont normally ask for moveset advice, I'm typically a pretty offensive person, i like to KO a Pokemon ASAP with moves that are super effective. Lately I've been staying away from moves with high damage but about 70 accuracy, my luck it keeps missing. Instead going with lower damage moves like Flamethrower, Thunderbolt, Ice Beam, etc. With Mew i feel since it can learn every move, maybe its too many possibilities and i get overwhelmed with choices, so I am asking for some advice. Mew is Lv 57 now and knows Psychic, Ancientpower, Dazzling Gleam, and Metronome, you guys have the floor now.
'Offensive' you say? Then you pretty much want to try Nasty Plot Mew (learned at LV 90). The only thing you need to worry about now is coverage.
One slot is reserved for Nasty Plot, the other one is for a STAB move, preferable Psyshock to circumvent the pink blos'. If we disregard past tutor moves, the best coverage you get is Fairy/Ghost - Dazzling Gleam and Shadow Ball. PlatinumDude suggested Ghost/Fighting which is okay, we think. Still, Fairy has super-effectiveness against Dragon, which we prefer.
