If Thanos has an attack of 0, do I still need a hero with the attack symbol to fight him?
The specific text for the Solo Mode Thanos variant reads:


  When fighting Thanos in Solo Mode and using a Villain Group besides Infinity Gems, Thanos gets -2 (attack symbol) for each Villain in your Victory Pile from that Group.


Firstly, if you are using the Infinity Gems Villain Group, this rule does not take effect, and his attack is only reduced by Infinity Gems.

Secondly, note that it does not state "every Villain in your Victory Pile", it specifically says "each Villain in your Victory Pile from that Group".

This indicates that you have to pick one Villain Group in the Villain Deck to replace Infinity Gems as his weakness.

Thus only one Villain Group in the deck would count against him, and since there are only 8 cards in any given Villain Group, the most that this could reduce his attack by is 16.