So there’s a lot in your post…so i hope my points don’t get confusing. And though you say you want to focus only on late game fights I don’t believe that is possible here because half of the solution to countering heroes like celeste and skaarf is the early game (I can’t speak for baron since since I don’t use him).
Anyway if you’re first paragraph you talked about how heroes like Celeste and skaarf can sit back and spam abilities with no care for energy.
Spammability and energy cost are exactly what make Celeste and Skaarf weak in the early and mid game. As a mage you CAN’T SPAM abilities without care during this time (this is why mages often buy clockwork or eve of harvest, so they don’t run out of energy late game). This is what you have to take advantage of as an assassin or jungler. As a bonus, mages are very squishy and have low mobility so it’s not hard to delete them as an assassin. Throw in a bit of help from a teammate and you’ll be fine. At very least you’ll do quite a bit of damage. Yeah you’ll lose some health but if a hero can basically delete others without losing health they would be broken. IMO once an assassin or jungler is able to close the gap whether it be late or mid or early, any mage is in big trouble if they don’t have boots up.
You also mentioned that it’s hard/impossible to dodge their abilities.
First I want to say that mages are heavily reliant on landing skill shots which is much more difficult to do than basic attacks or targeted abilities. Thus it would be expected that the reward for pulling it off is much greater.
Next I want to say that there are ways to learn to dodge and predict celeste and skaarf A abilities. Once you can predict you’ll it easy to dodge.
-NEVER stay in one spot. Do you know which heroes are the most annoying to go against as Celeste or skaarf? Vox and skye. Why? Because they always are moving around when laning. As long as you are last hitting keep moving around the lane before and after you attack. Any decent mage player will be keeping an eye out for when you are still because you’re an easy target.
-Celeste will always move forward to land her A because the range is short early-mid game. If you notice celeste moving towards you that’s an indicator that a star will be on your ass in the next couple seconds. Also if Celeste hasn’t dropped a star in awhile there’s a higher chance that she will soon (why? because we like spamming abilities, it’s a habit). As long as you aim to last hit, you shouldn’t have much trouble staying out of range and avoiding being hit.
-Skaarf’s A can be avoided if you pay attention to the projectile path. You always have to keep an eye on it and you can because it’s telegraphed as soon as skaarf uses it. Immediately go right or left of the path and buy T2 boots. They more mobile you are the better off you’re be. Like celeste, if you keep moving around, it’ll be difficult for Skaarf to predict where to aim the skill shot. This is why Saw is easy bait for Skaarf, because he very immobile.
Anyway my unsolicited advice was just to point out that it isn’t impossible and if you improve your skills a bit and start thinking of what actions celeste or skaarf will do next, you can outplay them. Remember that mages often look for weaknesses like immobility and try to predict your next move. Being unpredictable is key.
I was going to talk about the other things you mentioned but my own post is getting too long so i’m gonna end it here >_<