Yes on all of the above.

 I was saving gems to buy masteries but I ended up just taking a couple weeks of grinding Minotaur with free energy. I did my whole Demon Lord team at once and it was really gruelling but now it's done and I have over 5,000 gems! Buying masteries for the first few champions can be a good investment but it is possible to do masteries without gems.

  Ayumi Love, Hell Hades, etc are fantastic resources and some champions are more powerful than others but beating content is not as simple as six starring a bunch of highly rated champions and tier lists don't apply perfectly to this game. What is really important is building teams with champions that work together well to defeat the mechanics of the content you're facing. Whenever you're ranking up, gearing up or getting masteries for champions always think about what content you're trying to beat and who would work well on the team you're building. Sometimes your best choice might be a weird uncommon or rare instead of a shiny new legendary. Also just because one champion is the best at something doesn't mean another champion can't get the job done. For example I'm battling my way up Fire Knight right now and I really wish I had Alure to make this easier but since I don't I'm using Geomancer to keep his turn meter depleted. It's not ideal but it works and I already have him built for Demon Lord Clan Boss.

at this stage in the game, it's worth it. once you can reliably run minotaur 15, it doesn't make sense unless your scarcest resource is time. it's usually more cost-effective to buy energy and grind minotaur, plus you get silver and xp as well as masteries. but for the very early game, its a pretty good investment.

ayumilove is a reliable site, but i typically view it as more of a guideline than a step-by-step instruction. my champions usually look similar, but i can't remember the last time i didn't make at least some changes from the guide. most champions only have one or two setups, so they're designed to be broadly applicable. you certainly won't go wrong copying them, but you might not get the best build for your champion specifically.

with regard to kael specifically, his masteries seem pretty solid, but if you're using the cb and dungeon build (i would suggest that one), you should switch blood shield out for methodical.

I haave Valkyrie. No matter the team Valkyrie is in it...

Who has that much free energy to level up "niche rares"????? That shiny new legrendary wil lprobably help you out in more areea than the niche rare. I rather put Yannica to 60, which I can use in potion keep, Arenba, FW and Dungeons than that niche rare I can only use in one dungeon....

thanks everyone I very much appreciate your replies. Steve

You shouldn't build niche rares unless you have a reason to do so. But, for example, Master Butcher is a rare that helped a lot of people at Bommal. If you are stucked at a certain area of the game, a niche rare could be the way out.

A one trick pony like Master Butcher has less priority to build. The whole Doomtower has less priority. But maybe as lvl 60 champ no. 30 and 31, Master Butcher and Metal Shaper (for Scarab King) come in mind.

Totally agree!

Hello OP

1. if kael is your first not fully master'ed champion and you do not have mino 15 unlocked, then using 800 Gems is recommended

1.b However from your description you have described that you have 4x six star champions, which champions are those? are they able to beat stage 15 mino? if so I would highly suggest running them all 4 in mino however long it takes to fully master'y them, but I would check which champions they are first.

2. Put kael in lifesteal, he will benefit more from this once you give him Warmasters Tier 6 mastery 

3. I would suggest looking at early game guides on how to formulate a successful comp and focus on building a team of 5 

general advice is to have a:

x1 booster - this will allow your champions to rely less on the missing speed from your gear

x2 healer /buffer - this will allow your champions to sustain the hits from waves and bosses

dps champion - a champion that will deal most of the dmg in both the waves and the boss

wave control champion - this is a champion with a crowd control ability that forces the waves to not take a turn or direct their hits at them instead so that your team can focus on dps and healing 

(either in place of cc or the second healer) defence down on all enemies, this is to decrease the time it takes to finish the waves and boss

other things to consider is Atk down, for the waves/boss to take less dmg

good luck!

thanks everyone again Your advice is invaluable. 

I can't believe I did what I did.  I was fighting the Demon Boss and after , you know that another key comes up for 100 gems.  Well i'm on a laptop and must have touched the touchpad because I wasted 100 gems on a key. ARRRGGG , I would have been at 800 gems by now:(