Well it seems you can only run swords archers combo with Virion ressurect as free to play. 4 gold swords Frontline with 2-3 gold archers and 3 catapults... eventually turtle if you manage to borrow all that.
For pvp you need Calvary + Elena buff...and ton of heroes to outstat with Jack and Rose mandatory.
So overall forget about pvp and focus siege with swords and avoid lich garrisons.
As free to play best dragon should be something with extra building speed and extra research speed combined with building speed city hex. Rushing castle level and archers updated is crucial for human garrison early otherwise they fall short on DPS to do much. Then later you can switch to dragon with 2-3 passive purple buffs like melee troops hp, archers attacks rating and if you find one with 3rd for archers attacks speed buff...
At least that's the lazy low budget path I'm currently using .
For void swords and archers works well especially when ressurecting swords for those "no casualty" stages...For abyss you may need summon archers skill and eventually summon swords.
Base heroes are Avalon (hp+ archers buff). Virion (ressurect + swords buff+morale shield buff) . Vega (mass heal + casualty ratio) . Grim (siege skill for catapults) and eventually get Elena from chests for more casualty ratio+Calvary buff and other good skills she's got.
Also there are many detailed guides around forum you can check out as well