As of May 25th, your match difficulty is based on roster power of heroes you may not ever end up using. I suggest new players focus on the following heroes for PvP and hold off unlocking any non-essential heroes:
Kobal (only use is to die in PvP for disease, no runes needed except maybe some skill)
and a few others, as few as possible.
At the same time, limiting your roster will need to be balanced with Tower of Pwnage completion--and tower completion will be easiest with the above PvP roster equipped with strong runes.