[PvK]

(rhetorical question) How come all you have are big stupid troops as MA Agartha? When I play MA Agartha, I see good troops for conquering independent oceans (especially Pale One Soldiers in large enough numbers), which Vanheim generally doesn't do well, if at all. Meanwhile, their human infantry is also very good when used in quantity, if you have the patience to build decent-sized groups of armored men and deploy them well. All those units only cost 10 gold.

Some magical ideas: With fire and water magic and Construction research, you can give cheap Fire Brands and Frost Brands to your giants, enabling them to hit Vans regardless of their defense skill. Frozen Heart also ignores all defenses. Legions of Steel is a good choice to make it much harder for Vans to chop through your massed infantry.