Use their own magic against them.
Since the question assumes the existence of demons and magic, the easiest way to stop the magical demons is to fight fire with fire, by using magic itself. In a situation like this where humanity is facing the unknown, knowledge is power.
Send in heavily-armored robots equipped with metal nets to capture one of the demons and bring it to a laboratory for study.
If the demon teleports away, we now know that the teleportation magic that the portal used also applies to demons. How far away does the demon typically teleport?
If the demon breaks out of the nets, increase the tensile strength of the net until the demon can't break out; that way we can determine a rough upper bound on magically-enhanced strength.
If the demon is successfully contained and brought for analysis, take a tissue sample. Do the elements in its flesh correspond roughly with Earth's organic elements: carbon, nitrogen, oxygen, sulfur, etc.? Is its flesh any stronger than an ordinary human's?
If the demon can understand human speech and writing, what is its approximate IQ? Does it have superhuman intelligence, or is it essentially another animal?
If the demon can't understand human speech, how does it communicate, or does it communicate at all? Bring in another demon and observe their combined behavior.
And so on. One of the great things about the scientific method is that it can be applied to a wide variety of situations, even one where demons pop out of a hole and start attacking people. "For it is the chief characteristic of the religion of science, that it works."--Issac Asimov
Once the demons and their magic have been thoroughly studied, we can move on to actually using the magic. By what means do the demons use magic? Is it, say, by observation, by causing a quantum wave function collapse to alter the fabric of reality? (I'm just making this up as an example, I don't know if this is plausible or not.) Then, slowly, we can begin imitating the process of magic. Any sufficiently advanced technology is indistinguishable from magic; the idea here is to assemble the puzzle pieces that form the demon's magic. Step by step, we can begin to learn the ways of the demons, and once we've done that we can defeat them.
(Secure. Contain. Prot--wait, wrong website.)