Before P unlock to account lvl 100, I spent 800 gems for all champs. After unlock, I farm Minotaur for all champs. 

Reason: to get more account exp to reach lvl 100 sooner. 

Also I farm exp for champs as well, I upgrade a champ to 6 stars and start farming Mastery for them, normally I use 2 champs and farm exp/mastery for 3 champs. 

Oh, one more thing, I did calculate the amount of gems spending for Minotaur, if you farm last stage using gems, it's around 600 - 800 gems; 100% it's less than 800 gems. 

what do you mean "Using gems"???

I farm Minotaur to upgrade Masteries to 3rd or 4th level.  Then I play the champ.  If its as good as it should be I will farm more or buy gems.  

Too many champs look great on paper, but perform better/worse in the actual game.  

I have a different kind of parameter,

If you can do a run of Mino 15 in under 2 minutes per run definitely go for farming Mino 15

If you can do under 5 min but above 2 min, use 800 gem IF YOU LIKED the champ otherwise just farm mino 15

Above 5 Minutes definitely better using 800 gems.

It's one of the dungeons where there's no events and tournaments on, so i just think it's not worth the time to farm it unless I can do it fast.

1. Can you farm minotaur at stage 15 with 100% win rate? If No -> Buy with 800 gems.

2. Do you have at least 4 champions with full masteries? If No -> Buy with 800 gems. 

3. Are you ready to spend time running minotaur 154 times? If Yes -> Farm minotaur.

It's also good to start running minotaur when a champion is not level 60 yet. He gets experience too then.

I like @SunnyRay  chart ...

It took me around 3 days using all available energies to fill one champ full Mastery. 

In case I don't use "free energy", buy all energies to fill one champ Mastery, it's around 600-700 gems.

O.K. Thank you, guys!!!