best option to reach league 15 is really to get a good guild (always aim for a guild level 2 or above), try always also the check if their leader is actually active before apply to them (this will allow you to boost units by a lot (the cheap order unit buff at level 2 will give you something like 4 atk and 8 hp on each order unit, allowing you to easly go to league 15 in less then a week).
Also if you plan to spend some bucks into the promo packs remember : the offer will repeat till you will actually use it, but with different unit : if your new account has a starter pack (less then 1 dollar one) with a epic...... DONT BUY IT, just wait 1 or 2 days for one with a legendary in it (this worth for the 5 $ one and 20 aswell, no idea what other there will be after).
try to keep a friendly list of active people aswell, having people on league 12 will give you a chance to get epic from them for free (but any decent league levelled friend is good to have aswell, it may give you rares that you cant get yet otherwise ), it's true that you will eventually reach an harsh point where you gotta grind a bit of days before having a chance to keep raising on league, but that's not too bad if you plan ahead what you can promote and need.
both the referal and the referee get a mutual 10% of gem purchase from the other guy gem bought (this mean if someone start the game with your code and he buy the 0.99 pack with 450 gems in it, you will get 45 gems, but aswell each gem pack you will buy will credit your "apprentice" the 10% of gems you bought).