I've gotta say, after going through all 8 classes, I've gotta say Noble is the easiest. You're basically invincible if you can find yourself a pistol sniper (the base accuracy is ownage, and on top of that, you have your main skill) and a laser gun (For ballistas, knights, orcs, tanks, dragons).
Pretty easy on Medium. First I just build 2 shotgun's and upgrade them both to level 5. Then I build an electro tower. After that build more shotgun and electro towers until you have 3 of each, all at 5 stars. Save up and then upgrade 2 rocket launchers...if you get bored that is.
A suggestion would be making the damage colors different for different sides. For example, red if you're taking damage, green for ally's, white for enemies.
Also for 500 damage. You can use a bow that increases damage by 1000, make sure you stay back though and have good units (Great Hero's Ftw Located at: Grouzeno) to keep off the enemies. 500 damage should be easy with that. Then carry on normally.
To save, go into the desert (right outside a town counts) and click the wrench icon on the bottom right of the screen. It brings up a menu which includes Save Game.
My Guide: Start the game normally, but don't buy an extra hand or a cart. Just go on the trade route a few times with just a donkey, food, water, meds, forage. After the economy starts to worsen, buy a horse, stock up on food, water, and forage. Then make a run to Smerd/ Fort Goks/ Quabba. Make sure you have money left over or else the police will rob you to death. Avoid bandits at all costs. Once at Smerd/Fort Goks/Quabba, buy a cart and alchohol from Smerd. Sell the Alchohol to either Fort Goks or Quabba. Continue doing this because it's fast money and that means Hummers. Stick with the horse until you can buy yourself a Hummer or one of the 2 Jeeps. When you get bored of that, buy people and do the Bandit Hideout Quest and win the game.
My guess on easiest way to 500 damage. Pick Veseslav (the axe person). Level him up a few times, only increasing str. When you have 120 str (keep going till 200), buy the Elite 2H axe from Ra-Gorod. Then find armor that increases damage by 100-300. I don't recommend the stuff that increases by 1k because you die way too fast. If you have 200 str, Elite 2H axe, and 2 items with 100-300 damage, your attack range should be around 900-1200.
I find that conquering Grouzeno and buying Great Hero's is an easy way to win. I had conquered around 7 cities (All 7 were weak cities). Then I just bought 10 Great Hero's and I beat the whole game. I didn't need to heal them in any battle. I just stood back and let them do all the work. xD
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