Sentinel uses the fact that your opponents are limited in mana and goes well with the second card.
Canonist if you have an artifact aggro deck in your cube can really put a wrench in opponent's plan.
Vryn Wingmare is a second, worse 2 mana Thalia.
Archon and 3 mana Thalia constrain mana and are solid beaters.
The last two can be pretty miserable to play against, but I think Balance is a fun puzzle especially in planeswalker light cubes.