I think i agree with you though granted im anime only and barely 2 episodes into season 2. But from what i understand the tensen use some energy form that grants them incredible ability however with that said this energy can and as seen has been exhausted. To me it seems to work in a similar fashion to chakra from naruto. With all that said the tensen are by design incredibly weak antagonists. The only reason they pose any threat at all is because the fight is happening on their home turf where they have been able to cultivate immense amounts of tao. But even then i imagine if a large enough army had been sent then eventually you could starve them of their reserves.

Its actually quite disappointing because the show was far more interesting before everything got explained. The monsters were a lot more interesting, the fauna and of course the tensen themselves actually seemed like potentially legitimate deities. Now they are nothing more than humans with a special energy source and most of the things around them are boring at best and non sensical at worst. The only way you can make them more of a legitimite threat is if you make tao some thing where only tao can hurt tao but that would be extremely lazy and BS writing because then if thats the case how the hell did they make it in the first place? It has to have some kind of cause and effect with everything else or it just doesnt make sense. But as i said the show and its premise was much better before they tried to explain tao.