Word for physical resistance / resisting another's onslaught?

To mount a resistance is less ambiguous:

"Can the humans mount a resistance?"

"Can the humans mount an effective resistance?"

(The latter would imply that the humans may try to resist, but will ultimately fail.) It does have a strong military tone to it, which might not be appropriate depending on what these "Demons" are actually planning to do.

The first option is the same number of words; the second option adds one word but is less pithy.

You could use withstand as in "Are the humans able to withstand them?"

to stand or hold out against; resist or oppose, especially successfully:

Although it's not a single word, there is also stand up to as in "Are the humans able to stand up to them?"

To confront fearlessly; face up to.

Consider "endure", which implies that the demons pose a physical threat to the humans.

Endure — M-W

verb
: to continue to exist in the same state or condition
: to experience (pain or suffering) for a long time
: to deal with or accept (something unpleasant)

Another option would be "defend," although the clearest phrasing would require a second word:

"Are the humans able to defend themselves?"

Resist an attack made on (someone or something); protect from harm or danger.