War is hell. More so when you are fighting the Drowning Doom in the Sea of Black Tears in the Age of Metal, and they have the homefield advantage. I won the battle handily (after getting my arse kicked multiple times and nearly going bald with rage) and thus I have decided to pass my knowledge of how to complete the "Sea of Black Tears" mission with ease onto anyone who reads this.
Hold off on destroying the barrier whilst sending your units to attack any Fan Leeches on the field. Once a contingent of Grave Diggers crosses, take them out personally to give your men time to free up the Fan Geysers. Once you have all the Geysers free and Merch Booths set up, rally up all your forces at the opening of the bridge to cover the choke point (be sure to set up a Rally Flag for good measure). This stops the Doom from advancing too far past the bridge.
With the choke hold secured, you should now focus on building your army and raising the load to 40. Once you have a Rock Crusher, drive it backward and use it's double team to destroy the towers. Once they are both destroyed, accelerate and regroup with the rest of your army at the choke point. As the Doom approaches, start playing guitar solos like crazy. Shout a Battle Cry to motivate your troops, and use a Light of Dawn solo to remove any harmful enemy debuffs. Once they get close, you could also utilise a Bring it on Home attack for additional support. Your main focus should be to hold off their initial forces, which will include one Tree Back. With that first wave dealt with, you should make up for any casualties by recruiting new units as quickly as possible. Keep holding them off and pushing them back until you think you're ready for a final assault. When preparing for any endgame attack, a great deal of judgement is always important.
Good luck and give those gloomy mother fuckers Hell!