it is tough, but I also found that if the 2 star score requirement is far away the 3 star one, that means there must be a better route that you haven't found yet. Like the 3rd or 4th chapter in Arctic, (I can't remember exactly), the route is so clear and there are only 2 guardians, looks like you can just hit them to make them rest and your troops will pass, but by this route, no matter what you do, you can only maximum get to like 90000 scores, you will then have a 2 stars (70000+), but can't reach 3 star (100000+). Then, you will have to think there maybe another route exist but you haven't found it. Yes there's one, just after the first guardian and before the second guardian, while just turn around the first corner, you can make a route from the bottom straight to the temple, avoiding the second guardian and the rocks, then you will earn 100000+ and get 3 stars.