Ditto vs Raid Boss

Ooh, that's an interesting play. I need to start apparaising my Dittos. A perfect one might be interesting in raids.

Ditto's CP will be the CP of the boss with the same lvl/iv of Ditto.
For example, if you use a perfect iv Ditto lvl 30 against Lugia, the Ditto will become a Lugia cp 3084 (Lugia's cp at lvl 30 with perfect IV)

The ditto would be a copy of the legendary at the same level as the ditto with ditto's IVs, wouldn't copy the bonus stamina or level and ditto uses its own hp when it copies an opponent.
Not a bad choice if you have high level dittos and no better counters

Very good question!!! No answer from me.

The problem with ditto would be that most mons are resistant to their own type moves. So you would be doing 0.71x damage at best or maybe even 0.51x. Basically, Gengar and Dragonite are the only mons that take SE damage from same type attack moves.

If you knew that the raid boss had a move it was susceptible to such as Rock Smash Rhydon, but then you'd also be susceptible to the move as well.

So long story short, you'd either be spinning your wheels with using not effective moves (but you'd have less HP than the raid boss) OR you'd be getting hammered from super effective moves (again with less HP).