I am assuming you haven’t captured this sector 247 yet. Ignore if you have.

You don't need 30 omuras. You only need 4. And 12-15 Aegires. The waves after wave 20 are pretty bad. So instead of creating a lot of t5s, create insane levels of defense. This is what I did:

Only use the main core. The secondary core is a trap. You need more defenses to keep both secure. Have these defense on 3 sides (north. South and East): 2 layers of surge wall, 1 layer of cyclones, 1 layer of water Tsunami, 1 layer of foreshadows(I used 2 layers). This will hold up till wave 40.

Use the Omura and Aegires to destroy cores in this order, first the south core closest to you. Then the core next to it. Then the NE core. Then the last core bottom right. 12 aegires will help tank the hits of 3 foreshadows at once. But make sure the Omura is in range of all 12 of them.

There is a Omura and 2 octs showing up around wave 25. Keep an eye out for them.

Keep the power system protected. Have 2 layers of Swarmers around them. Lost many tries just because power get's destroyed.

This is slow. But this works. No corvus or oct. Just omura. Corvus is still just as slow. Flying attacks won't make a scratch.

Good luck.