Bullets are free, health is not
These two rules together are the whole game. You have unlimited ammunition, so firing costs you nothing and there is no reason to ever hold back a shot. Your health bar, on the other hand, cannot be restored by anything - there are no medkits, no pickups and no regeneration between waves. Whatever you start with is all you will ever have.
That combination is less common than it sounds and it removes a decision most shooters are built around. Normally you weigh taking a hit against the opportunity it buys, because you expect to heal later. Here there is no later. Damage is a one-way ratchet, so a hit you take at thirty seconds is still costing you at four minutes, and there is no play aggressive enough to be worth it.
The correct way to play, then, is unglamorous. Keep firing constantly because it is free, and treat contact as something to be avoided absolutely rather than managed. Runs do not end because you ran out of resources; they end because you accepted a few small hits that never got paid back.