You want to replace chao because his yellow tiles are weak against yellow opponents so only do half damage. That said, having three healers in your team may be a bit overkill (though it might actually help you considering your difficulties in surviving).
What items are you bringing? Are you bringing health potions?
When attacking the enemies, aim your hits at one enemy at a time. Take out the middle ones first, then the edges. Before you take down the last enemy of that round but after you get its health down, ghost tiles until your heroes’ mana are full. Then take down the last enemy. The next battle will be a lot quicker because you’ll start with your heroes’ specials.
Make sure you use health potions to keep your heroes alive and don’t be afraid to use damaging items, att up items, and other helpful items against the bosses.