If u use Q ability on Assassin, Blazer can't see u and panic bomb makes his flame deal almost nothing damage.
Gunner can see u before u use cloak and if u run in straight line, it's pretty easy to take u down as Assassin.
As Assassin, try to lock in then cancel right click when u use R ability. Hitting multiple scouts and then using E ability probably gonna kill them.
Shocker is anti-Assassin and if he is going from side to side, u should probably kill him mid-air and since orbs are slow, they are actually pretty easy to dodge. Their damage depends on distance - same as Rail gun; more distance, more dmg.
However, changing classes depending on enemy team is also good.
Commando can kill Shocker easily and also he is pretty much anti-Scout.
Assassin is hard counter for Blazer, Bomber is too.
Vs Blaster I would probably use Bomber since Bomber is great for taking down tanky classes.
Hope this will help.