While I 100% agree that the final stage feels impossible. It is the final stage. Made purely to give you a run for your money no matter how powerful you've become. Even when I survive the 2 hordes of enemies. It's usually a 50-50 whether or not I can actually beat the Heroine.
To try and help out, however. This is my strategy that has so far worked.
Place a goblin mage around top right corner at the start of the stage. Feed her what you do not feed the orca, so that you boost your kinetic attack. Make sure to have orca's energy maxed out before the hordes come. Once you see the hordes spawning in, focus on the necromancer summoners. They are your top priority, if you must focus on anything else. The trumpet lady is your other choice. Should you pick up any zombies while spamming your kinetic attack like crazy. Just toss them up in the air once. Then feed your zombie free pickings to the orca. No matter how fast and powerful you are. Enemies will start gathering up and attacking the Orca soon. Should all go well and no more summoners exist. Spam the orca's suction ability until the cannoner's arrive.
By this point you want to focus on sending out 2 specific allies. Centaurs and Kitsunes. Use the kitsunes as a panic button of sorts. Extremely cheap fodder meant for nothing but blocking 1-2 attacks and or a singular cannon shot. The centaurs however are made for soaking up attention and attacks. Place them in front of stray paladins and black knights.
When the heroine shows up is when things get truly annoying. By now you want to be almost 70% focused on her and her only. Always have 2 centaurs in front of her, if you can. Place them somewhat apart from each other. This can usually lead to the heroine 'wasting' an attack. As she only hits the 2 centaurs if your arguably lucky. By now you want to be focusing on slow but steady DMG. Placing Alraune and Mooru is the best for this. Do not overplay, however. As some more catapults will come along and gladly wipe them all out and waste your energy. So obviously, spread them out and perhaps even leave 1-2 close to the spawning zone. They'll get wiped out but at least it would be just that 1-2.
Once 260 enemies have been eliminated. You will have no more fear of stray catapults showing up. If you've done extremely well/got very lucky. Then you should be close to 300 max energy and hopefully a surviving Alraune or 3. By now you can either focus on trying to get as much reserve energy as possible. To continue spamming centaurs and slowly chipping away the heroine's health. Or you can try to get some last Alraune's out. I'd recommend energy. As you never know when the heroine will just simply attack very widely. And wipe out all those background allies.
So, you're basically looking to have a team of. 1 Goblin Mage, 1 Mooru, 2-3 centuars, 2 kitsunes, 3-4 Alraune's.
As a final note. I'd consider the kitsunes completely useless once they reach 160 energy consumption.
Hopefully this helps someone and I will go ahead and say. This is obviously not the most perfect strategy. Don't blame me if you try it once and still fail.