Because GURPS is a point-buy system, figuring out what constitutes as a “tough” opponent can be difficult. Not only difficult, it can be downright arcane. There are situations where a 500-point character can have barely any impact on a combat (lots of social and mental advantages), and a 100-pointer absolutely dominates it (purely combat-oriented advantages). There are no rules for doing this at the moment, just hard-won knowledge. That said, I do have a few guidelines I use:
- Figure out the minimum, the average, maximum amount of damage each character can do. For example, if a character has a ST 14 and their typical weapon is a Broadsword, their minimum is 3, average is 8, and maximum is 13.
- Take note of their best attack skill and Active Defense.
- Take note of their Hit Points, Damage Resistance, and any other defensive or combat trait (High Pain Threshold, Regeneration, etc.) that can affect a typical combat; which in my experience is between 3 to 15 rounds (3 to 15 seconds). Remember any equipment as well.
- Figure out what combat effects any exotic or supernatural powers might have on the combat. If a character can use Mind Control to make your opponents fight among themselves, this will have a significant impact on the combat!
Once you have all this information, do the same for any NPCs you’re going to use in the combat. Then go through the process above, but total all the PCs and NPCs on their side to figure out the approximate value of their collective damage-dealing, damage-taking, and chances of hitting their targets. Do the same for the NPCs. Once you compare the numbers you’ll be able to guesstimate how hard a given battle might be. For instance, if the PCs collective damage-dealing capability is 9 points (or about 2d+2) and the NPCs damage-taking capability is 7, it’s going to be a relatively easy battle.
Remember, these are only guidelines and the best determiner of how tough a combat is going to be is your experience with your players as their GM.