The problem of all this is your underestimating the power of melee !

Speed and strength isnt nothing IT S A LOT !
And Maybe his accuracy would be enought in close range since the spell are big !
But using sunch devastating spell in close range would Damage him also ! Making his hp drop even quicker !

For total anihilation ?
If seifer is close when he get the first hit !
Seymour jump !
And before Seymour do his attack Seifer can run behind Seymour to avoid his attack since Seymour doesnt have good accuracy and speed he wouldnt have time/accuracy to turn arround and do his attack !
So he would blow up all in front of him while Seifer would actualy be behind receiving no damage and he would also be Bashing Seymour !

More i bring stuff more it show how Seifer would Destroy Seymour ...