Build for a +def-spd Ophelia

How many mages you gonna put on her team?

I have the same exact one - and she's a beast!

You can go for the Desp. / Brash Assault Seal combo (similar to Micaiah) but I don't think it's necessary as very, very few things will survive a hit from her.

I ran mine with Life And Death 3, Chill Res 3, Res Tactic 3, Atk +3 seal, and Blazing Light when I got her and she did great work. I recently replaced her B-skill with Special Spiral after lucking into a Lewyn, but - honestly - even without it she's able to OHKO pretty much any unit on the roster with the right team setup.

Special Spiral is absolutely fantastic, but it isn't necessary for her to shine - since it changes her from a reliable OHKO canon to a full-on sweeper.

Edit: just realized it might be confusing as to why I'd suggest LnD 3 on a -spd unit. If you're going to use her for AoE specials, remember that invisible buffs (Death Blow, Drive Atk, etc.) don't count for the AoE damage. It's the visible attack stat that matters. I always forget that :)