How to beat Apollyon on your first try* (with military might (idk if genetics is good, it took me 4 hours to beat the game and I'm not replaying it)):
1. Greed heavily in the early game (sell your first core ~4 waves in);*resetting here is okay.
1.5 Spam retreat cool-down upgrade, and only use retreats when high reputation is at stake (e.g. you spent all your reputation on flamethrowers and you NEED them alive so you can get a refund)
2. Upgrade d-class whenever it's cheap relative to your income.
2.5 Alternatively, buy one upgrade of marauder collection so that you don't have to micro retreat d-class back before they die and save on retreats.
3A. Spam Axe Dudes (click macro heavily recommended) until they consistently start dying to AoE. Axe Dude Stacks are very good because they are extremely cheap to replace, allowing you to not bother with retreating them, and are additionally extremely strong until ranged AoE comes into play. The only units worth upgrading are (the rest are okay to upgrade, just not very important to do so): Axe Dudes, Shield Dudes, and Flamethrowers (the Fire resistance upgrade is at full effectiveness at level 4, so don't upgrade past there.)
3B. Keep upgrading your ability to buy better units until tier 9, then stop.
3C. If you get SCP-682, maximize the amount of time 682 spends on the field, but save retreats whenever 682 is out. 682 cannot be bought, but your other units can.
3C.5. If he dies, not much of a big deal.
(optional) 3.5. Invest in snipers and buy them for ice wolves/medium or big snails.
4. Late-game, just spam shields+flamethrowers+minigun in a 2:5:1 ratio
5. Defeat the boss (while minimizing the amount of frames sacrificed in the process) by spamming snipers.
6. Using these very effective (read: extremely simple to do, and mind numbing for the last ten waves) strategies, you should be able to do the "impossible finish"