Did you change Cecil's sword for the Legend, his first holy sword?
Make sure to do that, it actually causes MUCH more damage than that Silver thing = killing him faster.
Also save Tella for healing (Cure2 and Cure3 work good), while having Cid and Yang attack.

If you feel cocky, try throwing some Bolt3 spell or something with Tella (preferably late in the battle,
since chances are that the Dark Elf will actually die from that powerful magic blast),
but keep him mostly for healing!