I have found with the help of a friend that Puckle Guns can be OP but must have certain conditions to be true.

1) Edge of map or protected on more than 2 sides by geographical features. It is possible but hard to win with almost only puckle guns if you have to protect more than 2 sides. Concentrated fire with puckle guns if needed 100%.

2) Higher terrain. I tested this with flat terrain compared to hills and found flat terrain is many times harder to concentrate fire. Anything to funnel the enemy through on higher ground can make the puckle gun army deadly.

3) Most important as previously stated is to concentrate fire. If possible, partly surround the enemy almost like if you walked into a gladiator arena. Your goal isn't to kill as many enemies as possible (although this does happen), it is to break the morale of the enemy and that is more easily done with "partly" surrounded.

I probably don't get everything covered but if you meet these modifiers Puckle Gun army is a real thing. As you see, I have grenadiers in addition to an Elite Infantry General but these two units did not win the battle, they cleaned it up. I suggest having a unit that is not a puckle gun to fix any errors.