Zombie Rush is a wave survival shooter: the undead come at you in escalating numbers, you shoot them, and between waves you spend what you earned on better weapons, ammo and upgrades. There's no story to speak of and no puzzle — just the question of whether your firepower is growing faster than the horde.
The pacing is the design. Early waves are comfortable enough to bank money; later ones arrive faster than you can reload, and the run ends when you're overwhelmed. Every attempt gets a little further.
Keep moving backwards while shooting, funnel the horde through a chokepoint so they can't surround you, and reload between waves rather than during them. Upgrade ammo and reload speed before damage.
No — the waves keep escalating until you're overwhelmed. The wave number is your score.
Yes — it runs in the browser with nothing to install.