Try to attack earlier, and scout exhaustively in order to get valuable information about your enemy (types of unit, numbers, buildings). Use your obelisks and animals to scout. Also, try attacking first the core of his/her economy: create a raiding party and get those villies and ox carts (that'll soften him/her up before your main strike).
Early in a game nobody builds farms, so try and find those wandering hunters, dwarf miners and lumberjacks. A heavy hit to his economy (especially food) will weaken the whole advance. Constantly scourge those villies prevent the enemy's expansion, while keeping your own economy going. Set up trade only if mines are depleted.
As Loki, the enemy will rely heavily on Hersirs for shure, get them with infantry axemen, and pay also strong attention to throwing axemen. Build counter units, and bring in your priests and maybe pharaoh: Loki loves myth units. Micromanage them so that the TA troop doesn't finish your heroes before they eliminate the mythic menace.
Also, level down those Hill forts with catapults. Be shure the right unit is attacking the right kind of enemy. Remember: animals are strong and good support units but don't expect them to win a war for you.
Finally, if you get in a though situation late in the game, or for helping you finishing things out, rely on Midgol. These buildings should be preferred for city defense, instead of Towers (which are weak and rather expensive, and do not train any units). Beware of forward bases.
SPITFIRE
Let your rapidity be that of the wind, your compactness that of the forest. In raiding and plundering be like fire, in immovability like a mountain. Let your plans be dark and impenetrable as night, and when you move, fall like a thunderbolt.
- Sun Tzu, The Art of War