Ok, the numbers game!
It seems that all monsters of a certain type will always have the same amount of HP. So for example all fish have exactly 20 hp, while all creatures that stand upright have 100 hp. This is regardless of the planets threat level, a level 1 monster has the same HP as a level 100 monster. The difference is their armor rating.
Armor rating works in a ratio against your armor piecing rating, then multiplies your damage. So if you hit for 20 damage, and had 5 armor piercing, but the creature you were attacking had 10 armor rating, it would work out as 5/10 (Piercing over armor) * 20 (Your damage) and you would only hit for 10! In the same way if you were attacking something with only 1 armor it would be 5/1 * 20 and you would hit for 100 damage! This is why the difference between the first weapon you get and the tier 1 iron weapons is so huge.
This also means that to figure out your base damage is simple. Just multiply your damage with your piercing! Then to estimate how much you will hit for you guess that a creatures armor rating is equal to its level, and divide your base damage with its level. You can now see why lowering the saucers level by just one is actually a significant change.
This also works in reverse. You might have noticed that several different creatures will have the same attack. Body Slam is popular, but others exist. Each attack has a set damage, regardless of level, that is then multiplied by the creatures armor piecing rating, and finally divided by your defense.
The last small thing to note is that your defense is the average of your hat, chest and legs, not the total. Effectively this means that the armor rating of each piece is a third of what they tell you it is.