Among the heroes you have, the ones worth working on are
tashi driller lars octo volan volt
daphne bloodspear jasper mog optos
Boreas flora(till you get jasmine)
Among the current heroes, the best arena team you can make is
Tashi octo jasper mog boreas
Depending on stars and gear, you can switch octo with valan to protect your mog and Boreas.
For future, in addition to your current heroes get
Malrath cara tanya from Hall of legends shop
Zem Kong tut from Cof shop
Adeline vulko from Gladiator shop
Nilya from Arena shop
Mirielle tsuki from Guild shop
So for a f2p, one of the strongest arena team possible is
malrath valan tanya tut adeline
Hope this is enough to help you out.