What's wrong with levelling them up via "level up hero" or whatever its called? I level mine via TT to about 15 then pay for a big chunk, as far as I can afford generally. Too slow otherwise.
Nothing wrong with that way, but because it's so easy to get heroes upto lvl 25 with miminal effort, I reckon your are better saving the gems for your over 30 heroes as the XP needed from there on & up really starts to bite.
Aye I'm still in. Still play everyday & still not spent a penny. If you work the challenges you get plenty of in-game currency. Also finally started spending a few feathers.
The constant updates are frequent & actually improve the flow which is rare for a moby game. The latest update will make things a lot easier.
Working on 2 teams at the mo in prep for skill swapping.