They can be a real bugger. I have twin engines, so 12 injectors, 1-10 came fairly easy, 11 was real hard, broke the puller. 12 was a nightmare.
Do you have everything out of the way, the return line, etc. Do you have an injector puller ? Volvo Penta sells one that is poorly made and expensive.
Soak it down with a good penetrating oil and let it sit over night.
The injector has flat spots, a metric wrench will fit it nicely, sorry I don't remember the size. Get a small piece of pipe that fits over the end of the wrench to use as a cheater pipe. Try and twist the injector, bust it loose. I ended up ruining #12 but I did get it out. I though for sure I would tear the injector tube but it didn't. Twist it, more penetrating oil, try the puller. You can put hose clamps on the outside of the puller to beef it up, to keep the "legs" from bending, collapsing. Luckily number 12 was actually 2nd from front so it was somewhat easy to get at.
Did you get the cleaner brush that hooks to a drill for cleaning the tube after you get the injectors out?
Seattle Injector rebuilt mine.
Good luck, it's a battle.
Dope up the new ones with neversieze and get all new crushwashers .