This could be a little more interesting challenge, if you have to use only 1 hero, but you DO NOT know which will be the opponent hero.
So, noone will use a physical hero, as he would die against Spec.
Noone would use a tasty hero, as he would die against Spider.
So, many users will put spec or spider, because of the sure wins and careless players. So you need something that kills them both to start with.
Ninja would be a great start to try. But then, more people will start using Ninja.
So you need someone that kills spec, Spider and Ninja 1vs1.
this could actually be interesting gamble mode