Heavenfalls' advice is very good, but it does not tell you what to do about the dragon/shrill lord/drake that stomps your city on turn 50-100.
In my opinion, this happens at this point of the Beta because of bugs in the monster A.I. The developers are working on it, but right now, as a player, there is not much you can do if you have the bad luck to attract the wrong kind of attention early on.
"Not much" is not the same as "nothing". I have posted quite a few play-throughs (they are all linked from the last one), and in every one of them, you can see how I cleanse my homelands between turns 50-100.
The problem is that most of these tactics have been balanced out into mediocrity. They still work, but they require a lot more knowledge about the game to pull off.
Two of the strategies that still work:
A) Fortify + Dodge + Blindness
1. Play a race with Krax blood. You can customize it with "Lucky" for extra effectiveness.
2. Get a Death Disciple capable of casting "Blindness" Boost her penetration (spell mastery) as much as you can.
3. Train a hero or unit with high dodge. That is, either a sovereign/champion with monk robes, lucky skullcap, a shield and the dodge line of traits, or a trained unit with the balanced, lithe and arcobat traits, and a shield.
Attack the BigBeasty(tm) with the dodger, the dazzler and any ranged damage dealers you have. Avoid the BB until the dazzler gets "Blindness" to stick. Move the dodger next to the BB and fortify. Chances are, it won't be able to hit you much. Use all you got to kill the BB before it gets lucky against the dodger.
B ) Frozen + Slow + High Initiative damage dealer
1. Get a Water Disciple capable of casting "Freeze" and "Slow".
2. Train a hero or an unit with extreme initiative. A warg mount, the "Quick" / "Tactician" / "Swordsman" line of traits, a quick weapon and the armor of the gladiator will help you achieve that.
Cast "Freeze" on the BB while it's in your sphere of influence. Attack it with the dasher and the freezer. Avoid it until the freezer gets "Slow" to stick. Observe the turn list, and wait for the dasher to get to act twice between the actions of the BB. Dash in, cut it, dash out. It has been frozen, so it will not be able to reach the dasher. Make sure you don't get cornered, and eventually the BB will go down.
and of course you always have
C) Master scout
Play Tarth or a race with master scout. Your units and building will be somewhat safe from wandering monsters, if not from wildland dwellers. But seriously, you should never settle anywhere close to a wildland.
also, some people recommend the mystical option
D) have enough trained troops to overwhelm the Big Beasty
Those people are better players than me, because I cannot pull it off without crippling myself in the long run. Let them explain it