What's the easiest most efficient way to get them Koins?

Buy the broken PC version. (lol... seriously).

I think Towers probably but not sure which one is quite the best.

$20

I find the towers do a decent job, also just doing the Krypt tends to get you a decent amount of koins once you start progressing through it.

Max out a faction and the set you get allows you to get 400 koins per with an additional 200 for a faction kill.

Either way it's through your Kombat card. Just see which ones get you the most koins per match.

your kredit kard

Living towers I find are the best way to get a lot of coins early on, before you have access to some of the more abusable kombat kards. (That's assuming you can even access them of course.) I recommend getting the Perfectionist set by perfecting all the story QTE's. With the full set you get an extra 250 koins per match win, which is the most I've seen until you hit level 50 in a faction.

@el_funko just posted a good thread about farming Faction XP which also works great for Koins. with different Kombat Kard modifiers. Burality Kombat modifier in Kustom Kombat makes everything finish in seconds.

Bummer I didn't see this until now. The daily tower today started you and your opponent at 10% health. Takes about 6 minutes to go through and you can basically farm your little challenges to get a ton of coins. I've earned 90,000 in probably 20 minutes or so. Tower is still live for another hour, I suggest running through it with some bday that had an easy brutality (Erron black) and do his brutality and fatality challenges.