How to reduce difficulty? It's fun, but I keep dying constantly.
Reduce? On the contrary, I want to increase the difficulty, I went through the whole game on 20mm rotary guns at S-rank. The only problem was with the larvae, but they stop being a problem when you start to just collect them in big piles, circling around them and blowing them up with rockets. On the other levels, just take the 30mm cannon or railgun and shoot through the houses.
The 30mm GAU-32C will grind your enemies into dust, just point the barrel towards the enemy and hold down the trigger.
Smoke grenades work like a wall and turn off the brains of enemies on any mission except the 2nd. On the 2nd mission, just take incendiary grenades and a scatter gun.
Go, do a crime.