Ok, 1 star...
It is one of the worst grinding game I've seen in years.
Few tips to maybe improve the game :
1. as others stated already, remove the hourglass to allow grinding
2. If you insist on keeping the decrease loot on lower levels, at least increase by a ton the amount of loot collected, or decrease the cost of items/upgrade
3. If you want to prevent "passive" grind, give reward for doing mission instead of punishing people for not doing them. For instance place some loot cache on mission maps that players can collect during the zombie fight.
4. Give bonus rewards when people actively complete a mission with 3 star, to incite people to actually do mission and do them well.
Basically, stop punishing people and give them rewards instead, they will be more incline to play your game longer.
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