Introduction: (note this is work in progress) Edit
Hello there, if your reading this, then you are a top-notch high tier finn player, but you've seem to encounter some problems, or you need a new perspective. Well fear not, because now you step into a purple-tier finn player's guide :)
I like to refine and take parts of old guides that I feel are spot on, then will add or judge them to be more accurate for higher tiers.
"Finn's passive, Fury, amplifies your damage by a maximum of 160%, making attack damage very viable on him. Although the tooltip states only basic attacks and Bombastic Dash apply Fury, all other spells do too. Note that multi-target spells, which is almost all his spells, prioritize applying Fury to these targets in this order:
1: Any target who already has Fury stacks 2: First champion hit 3:First minion or monster hit.
Remember that Fury stacks reset upon a new target, so taking down one enemy at a time is more efficient."
Finn's passive is probably one of the best passive abilities in ATBP. This is the heart of finn. and I understand your thought process to say "attack speed is necessary!" However, it is not. (please see backpacks section if your overly curious)