For whatever reason, this game can be easier or harder depending on which cousin you're playing as. They're all different shapes and sizes, and this translates as them having differently-sized hitboxes in the punishment mini-game.
Myself and other players have had the most success with Cousin Drive - the boxy orange cousin who rolls around on a wheel. He appears to have a very thin hitbox, which gives him more chance of avoiding the falling rocks, and means that some rocks can appear to intersect him without connecting with his hitbox and scoring a hit.
The overall strategy is simple: head to the far left of the screen and stay there for as long as possible, only moving if you absolutely need to in order to avoid a rock. The random patterns of the rocks and Cousin Drive's smaller hitbox means you should have a decent chance of getting to 50,000 without needing to do very much at all.