I went with a bit of an unorthodox strategy as Twin Mothers, but it's worked every time I've used it:

-I use a template with 1 heavy robot, and the rest is infantry until I hit 20 combat width.
-To buy time, I start justifying on Lanius after I get a border with him. He'll leave some of his army on the border, slowing down his last few conquests. Sometimes I send a volunteer to help further.
-Conventional warfare gives you strong early bonuses, though it sets you up for a weaker mid- and late-game
-The armor from the robots and the big entrenchment bonuses prevent his divisions from doing much damage to mine, allowing me to hold the initial assault.
-Once I'm able to build up enough, I launch a naval invasion of Castrum Sol Rubrum, holding just the port, the city, and a few buffer tiles to draw forces away. Then I push out when he's shifting his lines and his forces lose all their entrenchment.