If you girdle a tree with a chainsaw do you need to spray it to kill it?

Just 1 cut around it.

These guys are spraying them. I am not going to cut 1,000 trees just ~25. Is there a premix that I can use.

I’ve girdled and sprayed with round up and killed them. But yes I would spray.

I've heard 50/50 glypho (41%) and water is strong enough but I usually go a little stronger just to be sure. Others might can chime in and give more info on the mix strength.
I also do a lot of drilling at a downward angle into the tree base with a 1/2" bit and then fill the hole with nearly straight glypho. Usually on thirds or quarters (3 holes or 4 holes) depending on the size of the tree. Girdling is easier to see if you forget which tree you have hit already and working over time like me.

If memory serves me correctly. 50% Garlon 3A, 10% Arsenal, 40% water.

That is the Harper cocktail. But add the Arsenal last after mixing the Garlon 3a and water. Garlon3a + Arsenal will gel if mixed directly.

I have the Garlon 3a and Arsenal on hand this year for a TSI project on my place but it will be hundreds of trees. Some quite large. Hack or frill and squirt, not girdling. But, I’ve had good success on sweetgum with a mix of 41% glyphosate, Garlon 4, and used cooking oil I had handy. The ratio was roughly 50/20/30. You have to agitate to bottle regularly though.

Do use proper PPE!