Mewtwo with Psychic (with full PP ups), Amnesia, Substitute, and Recover was literally unstoppable. Amnesia was so broken. Substitute protected Mewtwo from status changes and high powered physical attacks. Nothing that did physical attacks could take out 10 subs before being shredded to pieces by a maxed out special backed Psychic.
Make sub. Then amnesia. Replace subs as they break, using recover two turns after making the sub (always get the amnesia in first). When full health and full amnesia, unleash psychic.
For Mew, I always liked something more like Psychic, Ice Beam/Thunderbolt, Sub, Softboiled. Similar idea but can't get anywhere close to as OP as Mewtwo by abusing Amnesia. Good idea to wait until the 2nd sub breaks before using Softboiled due to the fewer PP of that move also. And full PP ups on Mew's psychic too...