I've got a few suggestions.
Try speeding up the attack animations, and putting the characters closer. Also, make changable characters, backgrounds and music.
But most of all, make the customization interface more user-friendly and malleable. For example, put the player and enemy stats on opposite sides of the screen as a reflection of one another.
Also, use two variables for each HP and MP value (e.g. PlayerCurrentHP, PlayerMaximumHP, EnemyCurrentMP, EnemyMaximumMP). This way, HP and MP values WON'T persist after battle, and it will allow you to have healing spells which don't give the player or enemy more HP than they started with.
Normal attack damage for both characters should have a minimum value and a maximum value, as well as a multiple which constrains the possible random values of attacks (such as multiples of 5). It's been a while since I worked with Flash code, but maybe something like...
ThisAttack = multiple*((random((MaxDamage-M inDamage)/multiple) + (MinDamage/multiple))
There may be errors in syntax, but I think you catch my drift.
Anyway, that's all the suggestions I have time for. Put them to good use, okay?