The latter.

Yeah, once you've started farming masteries, you're kinda stuck finishing... unless you just hate your gems and wanna crush em for fun. :D

That hurts now... Because I started to farm masteries for all my viable champs, so rationally I shouldn't buy them for any of them... But I just might do so anyways, cause I want to progress 

If you really want to progress, you will either spend money, or grind masteries.  Obviously.

Purchase them for 800 gems.

or spend about 680 gems for the energy required to grind them, and in the process earn about 1.2 million silver, as well as leveling your 5*or 6* champ to level 45.

By grinding, you save 120 gems, earn 1.2M silver, and get 45 levels on your champ!

edit: I may need to clarify, if you grind as energy regens naturally or daily refills, as I generally do, you can save all 800 gems.  If you start with no energy and purchase refills for 40 gems each, it will require 17, occasionally 18 refills.  You still save gems when purchasing the refills.  While there is variability in scrolls earned, it is remarkably standard over the course of taking a champ to full scrolls.  E.g. given 4 mini champs and one scroll champ, it will almost always require 156 to 158 runs, 2184 to 2212 energy, and 17 or 18 refills unless you just wait for regen.  Assuming you're hitting s15.

I can only do stage 13 with 100% winrate, also how do you farm it efficiently? I currently only have 1 champ with masteries, I put her in, and 4 others that I want to get the masteries for. Is there a more efficient way? I'm not at the level where I can build a solo Mino champ yet, tho I'm saving up my Shieldguard for that later on

So, efficiency is the thing.  At s13 it will take you longer.  Getting started is a very slow process and I did gem scrolls for more than a few champs at that stage.  But wouldn't waste gems on champs that already have a good bit of scrolls.

your first goal might be to get 4 champs fully mastered, that do well in mino.  Once you have 4 fully mastered champs to grind for one scroll champ, it will be much faster, but not necessarrily more efficient, as all the scrolls will go to the single mastered champ (faster), but they will only be receiving 1/5 of available experience, presuming your mastered champs are fully leveled.

at my level, I could solo mino with 4 champs grinding scrolls.  That would maximize experience and be more efficient in that sense, but would take longer, about 6-8 hours of constant grinding for me.  Result, about 630 runs, 4 champs to level 50 or so, fully scrolled, about 4.6m silver.  But I chose to use a fast team with one scroll champ.  Takes about an hour, only get the champ to 45.

If you have a champ ready to sole Minotaur 15...just put him and the champ you want to assign scrolls to. Forget about the rest.

Once you have advanced to a certain degree you might want to consider the strategy I am doing. Put 2-4 champs, fully scribed, who can run Minotaur 15 in under a minute and add the champion who should get the scrolls at level 1. Do not put gear or anything else on him. He should die with the first enemy attack to NOT slow down your manin team...he will get the scrolls anyway...

Who are some good champs who can solo Mino 15 easily.  Or duo even?

If you buy masteries for a champion that already has some masteries (rank 2-3) unlocked, the cost of those masteries will be deducted from the 800 gems. While searching for it on Google, I discovered the best small loan apps  through which I can obtain a loan. I no longer need to go to the bank to get a loan.

Just plain 'No'

For me: Ithos, other options: Theo, Ma'Shalled

Big'Un

Ninja

I used Ignatius for a while, and then eventually changed to using BEK.