It doesn't sound like a very bad idea to me. Maybe not a 15-20% increase though. Perhaps 5%. I don't think it'd be something that guarantees a win. Hell, the last time I had Titan HM in duty finder for trial roulette, 50% Echo couldn't guarantee it.

Still, the hard mode fight absolutely isn't insurmountable without any Echo at all. It is literally just memorization, and after a while your only problems should be the occasional mistake that might happen due to lag or some other unfortunate circumstance or the other people in your party. Extreme is about the same way, but its biggest problem is that any amount of lag is insta-death and a probable wipe.