Well pretty much each champion has combos. If youve ever played Mortal Kombat or games like that you know that some combos are harder to pull off than others. well think of it like that for champs. Fiz for example has a flip/double flip that grants immunity and a conditional dash to and enemy, an auto attak boosting ability and a projectile Ult. Well judging by this the ideal combo for him is Ult>Dash>AA>Flip>Booster>AA this combo reduces damage taken and capitalizes on damage dealt. not to mention its not too hard to pull off.
Leblanc on the other hand Has a dash, an on-ability hit damage mark, a chain that roots after a short time and a ult that mimics her last move. at first glance you can see that none of her abilities chain well into each other for combos and that makes her harder to play than fizz. not to mention Leblanc is squishy as all hell. with her you have to keep your distance from your opponent but still get in close for damage then get back out. while with Fizz, you can just go in and stay in.
Its factors like this that determine difficulty. to go even further you can find combos with other champions but thats another story ;)
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