How can my opponent cast so many spells on my units?
You have a pool of 500 favor, but it can be raised to 590 if you win artifacts (i.e. you were a member of the winning alliance on a world with Zeus' Spark).
You can also save up on items like "Blessing of Favor", which instantly fills your favor pool.
There's also the possibility to generate instant favor with heroes.
In addition, you can also obtain the ability to steal favor from other Players through the research "Temple looting", which is also instantly added to your favor pool.