Although he's just straight up horrible when it comes to getting you, Case is actually more of a lee than anything. Yes, he's the kind of man, who will fight tooth and nail for just about anything, but if he could purr, then he would while being tickled. He doesn't even know why, it just has a soothing effect on him. When you scratch his scalp, it produces the same effect. In fact, while he usually bashes heads in whenever he can, he'll be mostly still while you tickle him. It's not that hard to approach him, everyone comes to him for almost anything at this point, so he's used to it. He'll wonder what you need from him this time. If you've come just to play a little bit with him, then it actually makes him kind of happy. For the first time in a while, someone doesn't see him as just another soldier, but rather a buddy to play with. It warms his heart.
On a scale from one to ten, he'd likely be somewhere between six and seven for ticklishness. So not too much, but he can still feel it. His neck, underarms and thighs are some of his more sensitive spots. But in all honesty? He actually really enjoys it if you graze your fingers over his underarms. It gives him goosebumps. Yes, it does tickle and make him giggle a lot, but it feels so damn pleasant too, you know? He's not being attacked for once, someone is being gentle with him, what more could he want? If you have him pinned underneath you and go for his underarms, he'll actually raise his arms too to give you better acces to them. It heals his inner child and his touch starvation too. You can graze his entire upper body, if you're being gentle enough, he'll do his darndest to reveal more spots to you. He is feather ticklish, but something like a makeup brush works better on him.
Teasing is... interesting, in his case. If you threaten him with tickles in the sense of "I'm gonna get you and tickle you~" while wiggling your fingers at him, that's actually just an invitation for *you* to get wrecked instead. But if you say something along the lines of "Case, come on, your smile is so adorable! Your laughter is so infectious! Let me be the cause of both of them, please?" then it warms his heart and flusters him a little. He can easily say tickle, it's no problem whatsoever for him. He can even hear it without a problem, but someone being genuine when they say they like him for some of his features he can't really influence makes his chest feel warm. If you get some bare skin while delivering your promise then it's all the better too.
It really depends on how badly you've wrecked him whether or not he'll get his revenge. You were being mean to him? Just you wait until he gets you. He's gonna be a horrible ler and make you scream. You were being gentle with him and made him feel good instead? Most he's gonna do is provoke you into doing it again. In fact, if you snuggle up to him and then gently tickle him, he migh as well fall in love then and there. He'll growl and tell you to cut it out, but that's just an empty threat. Case is probably going to hold you closer to him and have you tickle him a little more. So yeah, if you're ever in a gentle ler mood, you know who to go to.