Let's not underestimate Erza. She's shown herself to be able to tank blasts which were said to be able to obliterate towns and she took 200x the damage she can inflict on herself when she destroyed Laxus lacrima orbs. She can tank pretty much any attack, maybe not C3 but definitely anything else Deidara can throw at her.
Then she has her flight armour which let her fight against Racer, before she got 3v1ed. Racer is the guy that can do this.
So speed shouldn't be much of an issue because with that armour on she may possibly be even faster than Deidara.
She also has lightening empress armour with which she can fire lightening beams or multiple beams of lightening which would make Deidara's C3 useless. It would also severely limit what attacks Deidara could use against her.
Even normally she can slash with such ferocity that she can rip right through steel similar to what Sephiroth was doing. In the manga just recently she took on 100 monsters, some of which were said to be as strong as the 10 great wizard saints.
Erza would win this in my opinion.