I have a free 2 play account as well (level 60)
My team there for campaigns is Valan, Kong, Taurus, Nilya, Gremor and before I was lucky with Gremor I used Tashi.
Tashi is pretty good for a free character (at later levels 90+++ he will slowly fall off) but since he is a front character it is important to atleast try and level him up to 5 stars. But on my f2p he is 3 stars atm and he is doing fine.
He is good for physical dojo, male dojo, he can be used on Firehawk in the prophecy pool (use his combination ultimate)
With an almost complete free to play account it is important to just focus on like 15 heroes. (you will need 15 decent heroes for epic arena)
Normal arena shop: Valan + Nilya move them to 3 stars, then 4 stars, then 5 stars.
CoF shop: Zem, kong, taurus, tashi, lingling, buy a lingling (dont level her stars untill later), get all these heroes to 3, then 5 and then 5 stars.
HoL shop: Tareth, Malrath, level to 4 stars,then to 5 stars.
Guild shop: Mirielle: level to 4 stars, then to 5 stars, Bloodspear can be good if you also like to go for damage in hol/raids.
Epic arena shop: Serafine, Garrick, garrick to 3, then both to 4, then both to 5.
This will leave you with heroes that should be able to complete all the dojos and most prophecy pools, the main issue will be gizmo, but you might have to rent a good dot hero from your guildies to keep it going.
Don't waste diamonds on anything other then your daily resets and in the case of an extremely bad hol run, buy an extra 5 attempts. Try to leech some 24 hour mines for a big amount of free diamonds.
EDIT: Osiris is pretty important character aswell and getting her to 5 stars by just doing her heroics on daily basis will help you with prophecy pool deathcrawler, polaris and also female dojo.
NOTE: female dojo requires a good tank, starting from heroic. (tareth and/or mirielle are your only options. (on free to play mirielle is a very important character to have as she will deal with the opposing annoying valans.)