Here's an easy strategy with bug Mews and Misty Terrain that works with 3 or 4 players.
You use 2-3 attacker Mews and 1 support Mew. Sets used were as follows:
Support Mew
Mew @ Light Clay
EVs: 252 HP / 252 Def / 6 SpD
Ability: Synchronize
Tera Type: Doesn't matter
Bold Nature
- Struggle Bug
- Life Dew
- Light Screen
- Misty Terrain
Attacker Mew
Mew @ Metronome
EVs: 252 HP / 252 Atk / 6 SpD
Ability: Synchronize
Tera Type: Bug
Adamant Nature
- Struggle Bug
- Leech Life
- Swords Dance
- Light Screen
Update: If you're willing to do a less basic set, 252 HP/ 208 Def / 48 Spe is better on Support Mew because it prevents you from taking extra damage or getting frozen before you can get Light Screen or Misty Terrain up. Attacker Mew should heal up to maximum with +6 Leech Life so the speed is less important.
Meteordash wrote up the strategy here:
Attacker Mews: First 3 turns, spam Struggle Bug. Upon unlocking your tera after 3 Struggle Bugs, you are free to SD 3 times. We tera'd as soon as we unlocked it, but for further optimization, its probably fine to stay Psychic type until you're ready
Support Mew: Drop Light Screen Turn 1, and Defense Cheer Turn 2. Proceed to Life Dew / Struggle Bug and alternating back and forth however they see fit. Support Mew never needs to click Tera as you're better off defensively as Psychic. Recast Light Screen whenever it's down!
If Mewtwo opts to CLEANSE ITS OWN drops, all 4 Mews begin to start spamming struggle bug until its -6 again for 1-2 turns. (You can have all 4 Mews Struggle Bug the first turn of cleanse, then have two Mew's struggle bug the following turn, and the other two Swords Dance.)
If Mewtwo opts to CLEANSE YOU, make sure its low on Spatk debuffs and proceed to recast your SD's! Mewtwo will do this MULTIPLE TIMES so be on the lookout!!
When Mewtwo is CLOSE to its shield breaking, every Attacker Mew stops attacking and waits for the Support Mew to Misty Terrain. This will COMPLETELY PREVENT rest from EVER going off! This means you can force the kill right away!
The strategy is super safe and you end up with a lot of time, since +6 SD and Leech Life essentially heals you every time. You can adjust the number of Struggle Bugs you do if you're not taking much damage. Here's how we finished with 3 people.
AJP explains this strategy in his video.
I assume you'd be able to get the Mighty Mark on a level 1 Mew if the 4th person idles, or uses cheers/protect/helping hand. If you don't click any moves, you won't die until it forces you to take a turn, and a level 1 doesn't dock as much timer.
Update: With a 2 attacker + 1 support + 4th low level Mew idling, this is still super winnable as long as you don't get haxxed. Here's my modified strategy, which I've been using to help people get the mark on low level Mews:
Support: light screen, then misty terrain, then struggle bug / life dew.
Attackers: 3 struggle bug, 3 SD, then immediately go into tera + leech life.
When Mewtwo cleanses itself, attacker mews keep doing leech life for the metronome bonus, support mew uses struggle bug while keeping terrain/screen up. The only time attacker mews change what they're doing is when they get cleansed.
The idle mew has protect in first slot and focus sash, which keeps it from dying enough to hurt us too much.
Here's how much time we were finishing with when we left the 4th one completely idle, and only used 2 attacker + 1 support Mew with the above strategy.