Legendary Pokémon

It looks good. Well, the last move will depend on the rest of your team.

Ok,I will go with Roar ,as I have Alakazam as special sweeper.

Hello! How about this Shedinja?

Shedinja
Lonely (+Atk, -Def)@Focus Sash
Wonder Guard
252 Atk, 252 Spd, 6 ??
- Protect
- Toxic
- Shadow Sneak
- X-Scissor

Focus Sash ensures that Shedinja will attack at least once. Toxic+Protect are really annoying against strong foes that have no ways to beat Shedinja (like Cresselia without Shadow Ball and Kyogre). Shadow Sneak is basically a STABed Quick Attack and X-Scissor has some power...
I couldn´t think on anything better. What do you think about it?
Ah, I almost forgot. Any attack boosting nature would fit Shedinja... the best one I´ve got is lonely, so it´s why I chose lonely over adamant

Pretty good moveset.I wonder if Swords Dance could do a work. For example, you put Shedinja against Cresselia. The opponent changes and you use one SD. The opponent attacks, you survive with the Focus Sash and you perform another Swords Dance. After that, the STABed Shadoe Sneak is going to be something really painful for the opposing Pokemon.

Yeah, I thought about that too... maybe Swords Dance would be better than Protect anyways.

Well, if you can clone your Pokemon, try more than one versions of the moveset in order to find the best one possble.

Hippowdon
Impish@Leftovers
252 hp/def.6 attack(Would 128def/128 attack be nice?)
Stealth Rock
Slack off
Ice Fang
Earthquake/Stone Edge/Roar

Is this okay?(I am not sure for the EV's though...)

Suggested Moveset:

Yawn
Stealth Rock
Curse/Stone Edge
Earthquake
Item: Focus Band

Nature: Adamant(255 Attack - 255 HP/Defense)

Looking at Hippowdon's stats, you can see that his physical Attack and Defense is good, but his Specials are lacking. This means that throwing Hippowdon against something like a Roserade (I love her) he's likely to get pwnt. I personally would rather have Stone Edge over Curse; if you're stuck with a Hippowdon with Earthquake as its only attack, you'll get pwnt. That's why Stone Edge is convenient. Earthquake is a no-brainer. Earthquake and Hippowdon's Attack is simply too powerful for most pokemon to stand. Throw a few Curses in there and Hippowdons is set.

Ok thanks for your advice fr3quency I will try it ASAP

I would use a more defensive Hippowdon with 252HP, 252Defense and Slack off istead of Curse/Stone Edge.

Good thinking, but what if the opponent used Roserade? In such cases, defense is useless.

If Roserade is sent out, just switch out.
Hey, a tip for you: never use numbers that aren´t multiple of 4 for your EVs. You know... each 4 EVs are used for something, so if you use 255 EVs, you´ll be just wasting EVs that could be used for something else.

4 EVs just give +1 in a stat. It's useless in other words.

It is not always useless. For exmple 1 more HP or one more Speed point can change a lot.

+1 in speed won't change anything if you are battling a Ninjask/Weavile/Crobat/Mewtwo/Deoxys etc. +1 in HP won't change anything as well. I'd rather use focus band. I cannot rely on that +1. +10+ is ok. If you think that a pokemon will become more powerful with +1 then why waste it? You could use that to gain +1 in Atk/def.