I wouldn’t recommend the putty knife, as the mating surface is soft aluminum and the sealing surface around each port is very narrow.
I had a lot of trouble getting the intake to come loose also. I believe it had never been removed in 90k miles. I used a mallet to hit it in all directions, but that didn’t do anything. I think the way I finally got it was to use a pry bar tween the upper half of the intake manifold and the front drivers side lifting bracket mounting surface. Prying in a way that rocked the IM upward. Obviously you want to be super careful as all the things you’re prying on and against are thin cast aluminum.
Another tactic that worked, once i got it started, was putting one hand into the hole in the back where the upper intake bolts on, and pulling up. Took a few swear words to get it though.