Unlock instructions, since TA is only showing the "after-pop" flavor rather than the pre-pop conditions...
"Your hero reaches level 30."
You will gain XP over time. At the very least, if you have a few minutes to burn off your rechargeable energy, go into the last map you've beaten, select a mode you HAVEN'T beaten (easy, medium, hard), and press
to Explore. This will give you some good XP in a short period of time. Towers are also good, and give you chances at better cards.