Scully goes "on strike" after anything she doesn't like, and gets seriously stressed.
With the SC fluids, what works for us is:
1 - I sing to her the whole time. This is absolutely essential. I even recorded my songs for the pet sitter to play during infusions when I'm not there and it made a big difference.
2 - I use a large (18) gauge needle so that it's very quick. She does much better with that than with a smaller gauge taking longer, even though it's more of a poke. But individual cats will vary in this regard. You have to see by trial and error.
3 - I don't physically restrain her. I just gently stroke her temples while singing and hold the needle in place, but it took me a while before this was possible. Initially I had one person gently hold her while I gave the fluids. Now she's used to it - doesn't like it, but is used to it and will stay still for up to 5 minutes provided I adhere to #4 and #5 below.
4 - I stop the infusion when she lets me know she's had enough (changes from mild discontent to serious protest, or tries to move away). This is very important both to avoid traumatizing her and because she often knows when it's starting to be too much. Even if it means having to infuse more often - or even twice the same day -- I find it best to stop when Scully indicates I should. I think this helps make the whole thing less frightening to her as she knows she has some control.
5 - At this point, 2 years into it, Scully will let me know when she's willing to receive fluids and when she is not, and mostly that's every other day. She lets me know in that if unwilling she runs off and hides or actively resists, and if willing I'll find her lying near to where the infusion set is and no serious resistance. But to get to this point takes time as in the beginning a cat won't be familiar with the whole process and how it makes her feel afterwards. With time she'll know that it makes her feel better and also when she needs it. The only times I've given her fluid when she seriously didn't want me to turned out to be mistakes and had adverse reactions.
Ideally one would give the fluids before a cat is experiencing anything that would let her know she needs fluids, because she's only going to feel the need when she's already a bit dehydrated. But in Scully's case I've found that it causes more problems than it solves to try to give her an infusion when she seriously objects. Other cats may be more amenable. It's a balancing act between the cat's physical and emotional needs. Both are important, as is your relationship with your cat.