Version 3

Now split the 12 coins into 4 groups. A,B,C & D.

Weigh A and B first, if A is heavier, we will then compare B & C, if they are equal, it shows tat A is the group that is wth a heavy counterfeit coin. Then we will take 2 coin from A to compare with the other 2 coins from A. The heavier side will then be split again to compare between each other to see which is heavier.

Version 2

Lets divide the 9 coins randomly into 3 group, A,B & C.

First we will take group A and B to compare. Then for EG. Group A is heavier then group B, so the counterfeit coin is sure to be 1 of the coins in group A.Then from Group A, we will take any 2 coin to compare the weigh, if 1 of them is heavier, that would be the counterfeit coin and if the scale is balanced, the last coin that is not weighed would be the counterfeit coin.

Then if the weight of Group A and B is the same, the counterfeit coin would then be one of the coins in group C. So, we will take any 2 coin in group C to compare the weigh, if both are equal, that would mean that the counterfeit coin is the last coin that have not been weighed.

Version1

First lets name the 8 coins,
A,B,C,D,E,F,G & H.

Now we will put A,B & C on one side of the weighing scale then D,E & F on the other side of the weighing scale. If the weighing scale is balance, then it shows that there is nothing wrong with coin A,B,C,D,E, & F, so the counterfeit coin would be either G or H if we do a weigh between this two coin.

But if the scale is not balanced when coin A,B,C and D,E,F is being compared, the coin on the lighter side of the scale would be taken out to find out which is the counterfeit coin.
EG. The total weight of A,B,C is lighter then the total weight of D,E,F, we will take coin A,B & C out. Then we will take any 2 coin to compare the weigh, if 1 of them is lighter that would be the counterfeit coin and if the scale is balanced, the last coin would be the counterfeit coin.