Keep distance. Dante is weak at long range, so try to keep your distance and attack from afar. This will help you avoid her powerful, close-range attacks.
Use status effects. Dante is vulnerable to status effects, so try to hit her with attacks that inflict these. Good status effects to use include poison, paralysis, and confusion.
Be prepared to heal. Dante can do a lot of damage, so make sure to be prepared to heal yourself when necessary. Use healing items and spells, and guard against her attacks when possible.
Summon Hohenheim. Hohenheim can be summoned to aid you in battle against Dante. Hohenheim will summon the other homunculi to fight alongside you. This can be a huge help, as the homunculi can do significant damage to Dante.
Stay focused. Dante is a tough opponent, so it is important to stay focused and not let your guard down. If you follow the tips above, you will be able to defeat her eventually.