Yeah the difficulty curve is a bit wonky. I've gotten 330 locally, that 400 built-in highscore is just speculation on my side lol.
The best way to get a higher score is to UPGRADE AS MUCH AS POSSIBLE, especially the blue turret. Each upgrade increases your range and decreases your cooldown a lil bit. Once you reach a certain point, a single turret can kill an enemy on its own.
Another tip: if your turret isn't shooting or upgrading, you're wasting precious time.