As a gunslinger who rolled from Destro since day one (please don't judge, I ended up hating my PvE dps back then) I can only say that tower of infinity just became a rockier road with each patch and subsequent nerf gunslingers got from them, first it was overly complicated aerial requirements that sometimes didn't trigger properly, after that we got some pvp skills to level our class (guard break on F, second spec AC which gave you 8 seconds of escape spam that resulted very useful in ToI and battlegrounds) but since gunslingers need to have some stupid requirement for a skill so they stay as gunslingers now we had to land Crossfire (4th key) so we could burst anything with our TAB (making gunners a single target burst class and therefore cutting a good chunk of their overall dps because of the time required to set up your burst properly), and now after ascension, bam, we lost what little skills we had to help ourselves in that damn tower, no guard break, almost all of our skills became blockable/resistable (yeah, even the TAB in aransu buff) and also we lost the second spec for AC (which made it useful aside from the fact that is just useful in parties/raids for resetting SB or revives mostly). It's been a sad and ridiculous story about how fire gunslingers became so gear dependent that you require TT gear to function properly, and that is without briging the ping to the table, anyways back to the issue at hand let's talk about ToI.
As a gunslinger the easiest classes for me to burst TF out of in 5 seconds or less are: summoners, warlocks, FMs(conditional)
-Summoners are a quite simple deal to do, you burst out the cat and then rest easy as killing the summ is pretty much mowing its HP down with basic attacks and explosions, nothing too complicated there
-Warlocks are essentially summoners, with annoying blocks mixed, but all in all after getting rid of the thrall (which seems to be the only thing in ToI that has a real CD) you can sidestep your way to killing them as long as you time your attacks correctly and manage to pull them out of sanctum and exhaust the bastion (don't count too much on the latter though, not sure why but sometimes bastion resists more than 5 attacks).
-FMs are as tricky as they get, before F80 you can easily burst them down and finish them with relative ease depending on your gear and how much HP you've left them with, but on the higher floors is where the cancer kicks in, they perma freeze your ass on the spot so you can't even get back on your targetting, and boy even as a gunner, who have a ridiculous amount of escape tools, FMs on high floors simply burn them as they simply land ice after ice right after your iframes from the hooks goes off, and if you're unlucky enough, they will simply stick your back to a wall with their infinite knockbacks and render you completely useless just so you can watch your HP go down as you curse yourself, you can only try and expect to be lucky and hit hard enough to burst a FM on higher floors, otherwise it's a lost battle.
After this there are the classes we start struggling with, that being SFs and Destroyers (another tricky opponent)
-When dealing with Soul Fighters since they lack the ridiculous perma block other classes have you can find windows to decently land your attacks or burst them leisurely, but you still need to watch out for their CC since you can get a rug pulled under your feet and well, that's not nice, it can be pretty frustrating.
-Destroyers are the start of that endless frustration you get as a gunslinger who tries to run ToI, not only the perma spin deflects ANYTHING you throw at it, but it also stuns you long enough for them to lock you into the cc and kill you in seconds, not even stall the timer because the spin gives them move speed and they usually take advantage of that to easily close whatever gap you put between him and your character, the only effective strategy I've managed to pull off so far has been starting off with a knockdown (recently for gunners it's tripshot: 1-->F, requieres a hook) and pray you can actually chain the rest of your cc while attempting to burst (just to clarify, twin barrels sometimes gets blocked, some times doesn't, same with haze bomb, still don't get it).
Now the classes that are parctically hopeless to fight against, and it's pretty logic since they're all melee, blade dancers, KFMs and BMs.
-Don't even need to mention why blade dancers get so frustrating, even more on higher floors, permanent spin and deflect, unbreakable chain of cc with included phantom grip, and if you manage to land a hit on them after burning off your resists and theirs, they will come back and stun you somehow with another deflect, it feels like there's no point in fighting them.
-KFMs are pretty much the same, but like @Lyn Thunderfox said, they will either block or resist EVERY SINGLE HIT YOU TRY TO LAND ON THEM, and proceed to cc lock you and make you wonder why are you even trying to aim high as a ranged class when you can't even keep your damn target because they will always be behind you.
-And Blade Masters... nothing to talk about here, permanent block, permanent sidestepping, perma cc chain with 50-90k lightning slashes that will practically make a fight have two outcomes, you either manage to burst them down in a single hit somehow (GC9 and full gear required) or they will burst you down with a few hits during the perma cc chain, the outcome just depends on gear and luck.
As a bonus mention, assassins are an annoyance of variable degree, if they don't stay permanently stealthed, they will keep you in cc long enough for the timer to run out or kill you fast enough so you can't recover, as a gunslinger who only has ONE non-target skill (I'm not even gonna count pvp skills because even though they show an effective range, you need to practically cast them at point blank for them to work, it's stupid but true) it becomes rather infuriating to try to remain in the damage area and try to lure the sin out, the only viable strategy you can dare to try is attack them while you're airborne and then go back to a big safe distance, wait for your skills to refresh and go again.
Only thing I can say is, don't feel to frustrated as a ranged class, right now the only way to get on a high floor is having end gear or being ridiculously lucky, nothing else will help, story is somewhat different with my melee alts, but it's still a gear check place anyway, the only good thing you can do there before hitting endgame gear is farming tokens/soulstones/sacreds, other than that, don't waste your time there.