Initially Pewter did have Golem support with his main army, but got greedy and split them off to thug as much of my land as possible.

Also notice it was a defensive battle for me, meaning he charged straight ahead into my biggest troop concentration.
As we said. rookie mistake. Does sound a bit like an amateur boxer being allowed to win, right up until the point where he find himself trying to headbut a uppercut.

Well yeah but still expensive and cap-only and suffer from nasty attriction. I got some to help expanding and although they did help, none survived early game. And remember I picked an air shield bless and everything. 80% arrows blocked still means 20% get through and they drop one by one, or if they get surrounded their defense wears out fast too.
Mhm.. thats why i didnt say OP as expander. But as counter to anything heavy.
I suspect a small team of them can murder anything up to and including a Lastygonian Tyrant not purpose build to deal with them.

Hmmm, if the units are fast enough they should only suffer light poison damage.

Actually Cerulean has managed to score a few wins agains Celadon and in those their armies don't seem to suffer any significant losses in the pursuit phase.
In this case they will be moving though several poison clouds. Im quite certain then, there is no such thing as light poison damage unless your poison resistant.

And yes, the relevant difference here though, is that Cerulean, as a Pokemon nation, are fighting the plants at a safe distance.