Not all of this can just be handled with slayer, some of this can just be with basic events. If you want tougher enemies to give more points you can just do an event when the bot dies to call the event. For example:
OnBotDeath > Client > IncScore
you would have to do that manually for each bot if you want them to dispense different points.
Now for the zombies to not all spawn at once, this honestly is a bit complicated since I don't use pre existing gamemodes I pretty much make stuff up myself for a game to work, so I can't really help there but I could make something up myself with relays.