1. Musclemass+charged cannonball will kill choppers in one shot on medium. In fact I don't think you even have to charge the cannonball up all the way. This lets you wipe out strike teams really fast, and when the choppers die they drop orbs that refill your health. On medium once I started cannonballing strike teams like that, they drop enough health that I could mess around near hives as much as I wanted. If you have access to blade air slice, get it, it will one shot tanks. If you have more than one hunter running around, abuse hunter dirtnap to keep things manageable.
2. The closest thing to a point of no return is the mission "Two Tickets," because after you finish it, there is no break between it and the final mission, so have your upgrades purchased before then. However, after you finish the final mission it lets you go back to the city and mess around, do side missions, etc. I will note that you should do military consume events earlier rather than later, because later on the military bases have more security, and it makes the "easy" infiltration events more annoying than they should be.
3. Ignore the marked targets and just spray dumbfire missiles into crowds. Just strafe over the water shooting into the street and such and go back and forth.
Here's a video that I used to get gold on it.
I don't really hold sprint down during fights at all - make sure you get all the movement upgrades, namely the air dash ones. Air dash often.
