i like it, and it actually took me a while to beat it, some tricks
your characters can lose all thei HP then heal some with their skill and swap for another before the defeat animation ends, this saves your character and you can use the defeat animation to gain time for the skills of your other characters, getting your skills ready is key, mammon specially is a great tank to gain time, you can let her get low swap and when she gets her skill swap back and heal
you should be changing the characters stats based on the use you will give that character that fight, lots of red enemies and your only leveled blue is khalom? then it might be better to focus on her offense than using her as support.
check their stats if they have lets say 0 in defense and 440 in hp, it will be better to increase defense than hp if you want it to last, focusing too much in defense and lifesteal can be bad too if your enemy is too strong it might be healing more than the dmg you do so look at why you are losing and try different things.
also you should be doing some already cleared maps in hard for extra points to lvl up.