The Seanchan have built a nice little empire by conquoring a group of weak, and disorganized countries. The Seanchan military had huge advantanges over Tarabon, Altara, Amadicia, and the Almoth Plain. All of which had weak or non existent national governments, and small poor armies. The Seanchan used overwhelming numbers, the raken provided intelligence and air support, and Damane which were not available to those they conquored.

The Seanchan generals are not Great Captains. Rand said the Ever Victorious Army won because they learned from their losses. The three times Seanchan army went up against a competent Randland general they lost. Rand about 5000 troops and a few Ashaman thumped the Seanchan. Rodel Itrulde destroyed the Seanchan army that was chasing him. Mat outmaneuvered whatever the Seanchan tried in Altara.

Egwene has assets than Tuon doesn't.
1. Linking which is unknown and unavailable to the Seanchan. Linking will allow Egwene's Aes Sedai to project more power at a greater distance than the Damane. Think a battleship firing its 16 inch guns at range that outranges a cruisers 8 inch guns.

2. Angreal and Sa'angreal. A weakened Egwene used an sa'angreal and was able to drive off the Seanchan force that attacked the White Tower.

3. Gareth Bryne is a Great Captain and is building a large army.

4. Allies. The Seanchan really have no allies. At a minimum Egwene can count on Elayne. Elayne the queen of two countries, has the Kin as power users, and cannons.

Tuon has one additional advantage. Egwene is focused on Tarmon Gaidon. Tuon is focused on the White Tower.

Egwene has one more advantage. Rand will not allow the White Tower to be crushed.