Even 400$ and 100% sail action for first purchase can end in max 14 or may be 20 mio gold. And this is probably absolutly maximum for any buyer. Only free stuff member have sometimes 20 mio+ gold.

Average gold user buy first purchase or blackfriday sail on steam 10$ till may be 100$. that mean 150k till 1.5 mio gold.
150k are nothing for good teamplay of opponents. 1.5 are may be unstoppable, but fighting that ammount make you more experienced and keep other rounds gold free.

Also i hope really you are on version 1.5, so you can slow down his troops by damaging with artillery. He could heal it. but it is expencive. several heals for full stacks can devasate his gold reserves.

let 1 till 5 own unit far away for case you lose your cores. on day his gold state reserve will be emty your coaltion can reconquer your cores, but you need to be active and have at least 1 land unit for reconquerig by yourself.

build several 10 regiments strong divisions. use them near each others. If your enemy has airforce or artillery you should use your forces in closecombat reachwide of pixel on map. So inflicted damage will be disperced by all your troops.

For the case that single divisions merge automatical, dont let them do it, by giving diferent orders and diferent time delay. As long you are active you could fire your artillery manual after each other. (Division of artillery, not single regiments, that would be too time intensive)

simultanous attack on core of your enemy by you and at least two other allies. Killing ressources of enemy your raise his gold wasting for new units.
Such sneaky flanking actions should be while enemy is offline.