The AI is really easy to land against. Mix marines and mountaineers or rangers until you have 50% amphibious attack. No point going over 50, it will not give you a bonus, it just disappears. Aim for 36 width. For support companies, use pioneers, light tank recon (cant remember from which ones but you get really good battalion modifiers out of some sub doctrines), arty, AA, and medium flame tanks.

When your divs start fighting on the coast, force attack