Got this idea from one of the use maps in warcraft.
So we have the base in the center, or in the corner, the viruses will be approaching to the base
The aim is to defend the base for a certain amount of time, the mobs get stronger as the time passes
The players will get 5 lives, and they can use their mercenaries with they want.
Their will be a boss for every one minute, if you kill it, it will drop small awards such as 1-day items.
No matter if the players cleared or not, the players will get xps depending on the number of mobs they killed.
However, if lost, players will get 1/4 of the xp which they should get.
Any suggestions are welcome!