Actually, reducing damage across the board is a viable solution

Provided that the damage reduction is proportionate to the appropriate amount of damage a particular class of champions ought to be outputting.

Of course -10% on all classes is going to affect certain classes more than others and isn't necessarily the best solution, but League ALREADY has an entire system dedicated to categorizing champions that could be utilized to tailor damage reduction across various classes. Damage reduction does not have to be EQUITABLE across the board, but it can certainly be across the board nonetheless, and it SHOULD be.

The Assassin class can be nerfed in a way that is not necessarily similar to the marksman class or juggernaut class or skirmisher class for example.

Will it be perfect then? No, but it won't be anywhere near as potentially catastrophic, and the balancing out will be much more manageable from there.

This of course, can occur in conjunction with changes to runes or items, but I truly believe that directly targeting champions would be much more appropriate.

Look at raw numbers in the later end of season 5 when he was considered overpowered. Compare him to the current "undertuned" darius and you'd easily be able to see that the current darius would absolutely stomp the shit out of season 5 darius just by the raw numbers alone, and certainly after next patch even moreso.

Damage creep is like debt, it's incredibly painful to deleverage but you have to do it at some point. Making excuses for why not to is only going to lead to more damage creep in the long term until it becomes completely unsustainable and the game is horrendously unfun to play for the vast majority of people.