First thing to do is bind your most frequently used ability to MB4 (most meeces have an MB4), i.e. the thumb button. That alone solves a lot of potential RSI problems.
Next thing is to learn to mousesteer. When you hold down right-click and move the mouse around, and then press a movement key like W you move in whatever direction you are facing. I can’t remember offhand whether GW2 defaults to S and D as being side-step or turn, but you want them on side-step, so you can eventually circle-strafe. You might find you need to switch the Y axis (as someone who cut his teeth on Descent, I got the sense of “forward=down” ingrained in me). Most people are more intuitively at home with “forward=up”, so the effect is like directly painting on the screen.
Next is you need to learn to be able to use two fingers on your left hand (! :) ). You will generally be pressing either W, A, S or D while steering with your mouse. That means you have to learn to be able to press an attack key with another finger, while holding down the movement finger. So for example I am side stepping with D using the forefinger and hitting 2 with my ring finger, or 3 or 4 with my middle finger; generally 5 is a bit awkward for me to hit with my middle finger while holding down D with my forefinger, so I just switch circle-strafing, and hold S with my ring finger while hitting 5 with my index finger.
It sounds complicated written down, but it soon become second nature. The foundation is basically to learn to use the fingers in “rows” and “columns”, just like when you learn to touch type. Each finger has an assigned “column” of keys that you only occasionally need to deviate from.
Another thing is to use keys adjacent to WASD for many of your key abilities. For example I have the class function keys assigned to Q, E, R and T. I also have Utilities assigned to X, C, V, B and N (the forefinger is probably the most articulate finger and can do a lot) and loot/interact to F (mass loot to SHIFT+F). I have “switch weapons” assigned to Z (so the ring finger has easy access to it). But YMMV, and you have to experiment to find what’s comfy for you.
The next thing is that for combat like GW2 you need to be mobile, and you have to circle-strafe, and you have to dodge. Circle-strafing you probably know already from fps-es (if you don’t, there are plenty guides online), the only difference is that you are holding down RMB for mouselook rather than having “free” mouselook. You can dodge by double-tapping a direction key, but I personally find that awkward so I bind SHIFT+MB3 (middle mouse button) to dodge. You can use any keypress combo (and have double-click to dodge disabled) so you can find some way of making it comfortable.
Camera-only control is on LMB by default, and I leave it there for when I just want to look around while my character is moving. (Autorun I have keyed to G and use it all the time.)
Now as to GW2 combat specifically, the thing you want to watch out for is how different weapons and different abilities synergise with each other. It really pays to absorb what the tooltips say about abilities, and experiment with easy mobs or dummies so you can figure out rotations and combos. One of the most notable things in GW2 is “combo fields” and “combo finishers”. These can work for yourself or for allies (you lay down a field, they exploit with their finishers, or vice versa). For yourself in solo play, often you will find that you can do some neat tricks by laying down a combo field with one weapon, and then switching to another weapon with a combo finisher. A big part of gameplay for many classes involves frequent switching between weapons to exploit combos, or conditions.
