"Complete the Leviathan raid or a Nightfall strike on Prestige difficulty."

As written, this literally means you can get the trophy for:

Beating the raid

OR

Beating the Nightfall on prestige.

Given that beating the raid is a separate trophy, what I think they meant was:

Beat the raid on prestige

OR

Beat the Nightfall on prestige.

So how should the trophy be written to be correct grammatically and unambiguous?

"Complete the Leviathan raid or a Nightfall strike, on Prestige difficulty."

One comma added. Should be fixed in the next patch :-)