1- This level is the lowest tier, and as such, those belonging to it are not especially challenging. Enemies encountered at this level can, and dare I say it - should - be dropped in a single hit. They can defend themselves just fine, and cause problems if given the chance, but in general, these enemies are grunts that go down without much trouble. Goblins, large spiders, lesser demon foot soldiers and guard dogs fall under this category
2- This level is a distinct step up from the bottom of the barrel. Enemies here can take a few hits, and should not be treated lightly. One-on-one, they might not prove to be too great a threat, but they can still give PCs a run for their money. Their attacks pack more of a punch, and they might not drop in a single hit. However, they are ultimately not something that would overwhelm the party, unless in large numbers. The average city guard, a goblin berserker, a dragon hatchling and hell hound are in this category
3- This level is the middle tier, and is essentially where the PCs can view themselves, for the sake of relating to their opposition. Enemies here are as capable as the PCs, and battles are likely to be hard fought. Enemies here most certainly cannot be felled in one strike, unless a situation is presented to the contrary. They also have magical and martial prowess on par with that of the heroes. An army general, a young dragon, a troll and an angel fall here.
4- This level presents a serious challenge to the heroes. Enemies found here can be expected to have either superior mastery of spellcraft or martial combat, but perhaps not both. The enemies here can be expected to put the PCs on notice from the get-go, and they only become more dangerous with minions by their side. PCs can hold their own against them, but not one-on-one, unless it's a special situation. The high mage, vampires, minor demon lords and most elementals are of this level.
5- This level numbers the best of the best amongst its ranks. These enemies can single-handedly take on the entire party without fear. Their abilities far outmatch any individual PC, to the point where it is literally only the combined effort of the entire group that can hope to bring down this threat. Even with concentrated effort, the party is expected to get tossed around quite a bit, and at least some will sustain serious injuries. These foes can not only push the party to the brink of defeat; they can very likely hand the heroes a stinging loss, based on the party's previous decisions. Elder elementals, the arch angel, demon lords and an adult dragon fall under this category.
6- This level is reserved for gods or creatures with unlimited power. If you're encountering one, hope that it's benevolent.
So, if the group has encountered a level 4 enemy, don't just one shot kill the creature. There's a reason you're encountering something that tough and I'm trying to use it as a plot device.
Finally, when combat is joined, I'll post something like this:
Enemies
4x Giant spiders, lvl 2
1x Queen spider, lvl 3
1x Dark robed spellcaster*, lvl 4
1x Queen spider, lvl 3
1x Dark robed spellcaster*, lvl 4
If there is no asterix (That little star thingy for those who don't know the name of it), then you may control the enemies actions, kill it, dismember it, capture it, do as you wish within the bounds of the gaia ToS to it. If, however, like the afore mentioned spellcaster, which has an asterix, you CANNOT do as you wish. You can have some limited control, i.e., trading blows or spells, it taunting, etc, but when it comes to things like vital wounds, capturing, etc, those decisions rest with the person in control of the thread, and them only.