While this page is named Monsters it mostly talks about PvE combat.
The combat should be action-oriented and involved. It should also be heavily gear-dependant. Level doesn't mean a whole lot, its more about challenge.
A good starting point would be to borrow from YoF with the Star and Color System.
Essentially, each monster will have a Star Rating and a Color. Star Rating is over-all challenge while color is relative strength within it's star rating. Players could have a Star Rating of their own based on stats, perks, and gear, which determines how effective they will be. A player might be able to take on a weaker monster who is one or two stars above them, but may not want to tangle with a monster that is even their own star-rating.
We could also have monster stats adjust dynamically according to who is attacking them. Lets say, for example, that level 5 player has teamed up with a level 50 player, things could be adjusted so that they do basically the same amount of damage. We could also forgo the notion of monster hit points all together.
Attack and Defense
Borrowing the "There is no 12" system from Year of Fire.
Effectively, everything has attack dice and defense dice. On attacking, an attacker must first bypass the target's defense.
There are four types of hit points: 0, .5, 1, and 1.5, each corresponds do a different type of crit. 0 is a regular hit, 0.5 is a half-crit, 1 is a crit, and 1.5 is an overcrit. So a hit point pool might look like: 0, 0, 0, 0.5, 1. So to kill it you would need to hit, hit, hit, half-crit, crit.