I did a lot of the easy missions/quests/challenges to stack up on energy, mostly. The energy required to complete them was less than what I was given for doing them. Focused hard on getting my starter (Athel) to 6* while leaving the rest as 4* so that I could farm 12-3. Think it took a while even so to get better gear and manage brutal, but for the most part I was using the free lifesteal set on her for a very long time. I did 12-3 hard meanwhile, and initially with a support (think I used Warpriest) until Athel could do it on her own.

My suggestion is to focus on leveling solely.  I am f2p and early on l focused on Tournaments to get ancient shards and Events.  This help me back a bit.  But once you get your first level 6 to farm 12-3/12-6 you will find that leveling up further level 6s comes much easier.  That said, l'm slightly over six months in and only have 14 level 6 champions.  Hopefully you will progress a bit faster than me.  :-)

It's pretty simple - put the lifesteal gear from login rewards on your starter champion and get them farming the highest possible campaign stage, brutal 12.3 as soon as you can, then do almost nothing else than farming XP, your starter plus 3 food champions. Obviously with double XP bonuses as often as you can get them - times when you don't have double XP are when you could do minotaur or gear dungeons. 

Getting a 6* within the first week is pretty tough and I think only veterans on their second account really achieve that, but if you're more than a month in and still don't have a 6*, that's because you weren't trying hard enough - making too many 50s instead of going for your first 60 first. 

several bucks and you will have more than cuple of 6* in a week

I'm F2P lol

I started again several accounts, reach lvl 30 in 1 day, sound good ? till I stuck at silver mission (seem forever), I quit all those accounts.