I figured out a good way to tie him up outside, so I gave it a try yesterday - he was such a good boy. 
Now that I have a system it’s no problem, I don’t think.
I second the recommendation for the booster bath. I got one for my in-laws Great Pyrneesx4 and it is a life saver. They were previously bathing the dogs half at a time in a big round washbasin. Total suckage.
That said however, I usually bathe my dogs in the bath tub. Brute force is the answer, and even if you are not large you can move a dog even if they do that “YOU CAN’T TAKE ME! I HAVE NO BONES!” thing that they are so fond of. Once you lock yourself and the dog in the bathroom the rest is gravy. A massagey showerhead is great, but you can also do this with rinsing with a bowl. Bathing with two people is easier, then you have one person who’s sole job is to hold the dog. Prepare to get absolutely soaked, and bring lots and lots of towels. No matter what the dog does do not get angry and yell. This will only freak the dog out. Always be in control and speak in a firm tone.
It also helps to sing “Keikei the good dog” to the tune of Frosty the Snowman.
Or at least it did for me.