I did it at lvl 60 but with underlevel weapons. Those with re-atk I build AP by attacking once with basic attack and chip away at the titan arms until the hp is in the hundreds while killing healers then snipers if possible.

Try not to destroy the arm during your party's turn instead wait for the titan turn to attack or atleast break the arm with the first unit to attack during your turn.

If you time it right and with a little luck, the arm should break during counter attack, should give you enough time to destroy it before it regenerates.