Ok for the ipad account...
You want to lead with Indra for now... put the different dragons you have on your team (try and get one from each color for now) and while you are levelling up try to farm a healer girls team...
Echidna, Mermaid, Lillith, (Angel and Alraune are not very good) They all drop from Stage 2 of the Tier 2 Dungeons (Sea Cave Mercury - Voice of the Siren for mermaid, Fire forest Nerva - fire worshipping snake for echidna, and Twin Temple Diana - the coaxing snake for Lillith) You have three toy dragons (shardra, toytops, pierdrawn) they are situationally useful... dont bother levelling them up until much later...
You also have a noel egg and a mystic red knight... I am not sure what color the noel is... but he is a high hp high rcv dragon with pretty decent stats... Apart from Valk, and the starters... you do not have a real mono color leader... so I would suggest rolling during the god fest today on that account.
As for the phone account, Persephone is THE dark leader... so grats on that... and you already have a lillith too.... Now you just need a CDK, Vampire and black ripper and you have a great monodark team!
A mermaid will always be useful for every account so I suggest working on getting one of those as you level up... Sea cave mercury - voice of the siren
And you can find vampire at castle of satan when you get that far...
The strategy for levelling monsters is pretty straightforward...
If its Red Blue or Green... you feed your target with Carbuncles. But before you feed the target, you level the carbuncle to lvl 12 or so using stuff of the same color... then feed 5 lvl 12 or so carbuncles to your target.
If its dark/light you feed off color stuff to your dark/light carbuncle and do the same thing... this strategy gets you to castle of satan...
As for choosing what to level... your persephone has already outlevelled your other stuff by alot... so dont waste more gold feeding it junk for now... feel free to evolve your red starting dragon one more step and get him to lvl 35. After that, focus your red stuff on your lil red dragon and work on evolving him to max (he is a red ripper)
If you need more specifics... just post another question