"Please help me with my team:"
Mewtwo @ miracleberry
Could be banned, icebeam over punch though. The ice move on mewtwo isn't my favourite, I would rather see flamethrower over it. Good choice of psychic though - it keeps mewtwo authentic and is very powerful. It sounds weird, but I think you may be able to use this if you drop recover and give it safeguard/amnesia. If it is totally banned, Mr.Mime is an ok replacement. However, I will warn you that dark types aree becoming increasingly popular as starters - I have seen quite a few houndys/tyrans lately.
Octillery @ Quick claw
A fine choice. However, you can make this better by giving it flamethrower - as silly as it sounds, it's possible! Fire/water make a good combo, and you already have icebeam on mewtwo, so if you want the psychic one to keep it, change icebeam for flamethrower. Otherwise, fine. Return is good as it has a great attack.
Raikou @ Brightpowder
Good, it is legendary, but I don't think it is standard. Anyway, thunder is a must - it is a pokegod (kinda)!! I like the bright powder here, not the best universal item, but still can work.
Tyranitar @ Leftovers
AP is a better choice, though if you choose that attach the mysteryberry. Crunch isn't necessary here - already have it on Raikou. If you want to keep a dark attack, pursuit is great, for the psychics. Here's a trick. When you see a psychic, say alakazam, switch in tyran. They will choose to switch, and you will get a free hit. I would be inclined to give it rest over quake or fireblast, as with AP boni, you want to keep it alive for as long as possible. Thus:
Snorlax @ mintberry
Yeah...can't see it working pal with roarers etc. If you must keep him, unleash him as the last one, where he can't be force-switched. To tell the truth, the best thing you can do with this poke is to have lots of attacks and just go on a rampage, say:
Dragonite @ dragon fang
To tell the truth I'm doubting it's usefulness. You already have ice/electric support, and it is mewtwo/raikou who will use those attacks respectively better. This team lacks a fighter, and thus a decent way to give miltanks a good kicking. I would recommend getting either hitmonlee/top/chan in, or primeape. If you have to have it, Machamp, but give it a weird set.
Overall, I can see this team doing well, but apart from octillery, who is only quasi-uncommon, the others are all really popular, you have BOTH tyran and drago, two legenadaries, and no pseudopassing support. This team can really become unstuck vs a parafusion team.
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A legendary is only legendary if it has a legendary set.