Subterror Behemoth Umastryx's effect is an optional effect (You Can) that starts with "When" which means it will miss activation if another event happens after "When a face-up monster you control is flipped face-down". Subterror Final Battle won't trigger Subterror Behemoth Umastryx's effect. Its text says "Activate 1 of these effects, also, after that, Set this card face-down instead of sending it to the GY". Because the text has "after that", another event is taking place after changing 1 face-up "Subterror" monster on the field to face-down Defense Position. The conjunctives "then" and "also, after that" indicate that the part of the effect before it is done strictly before the part of the effect after it, and so effects that could be activated in response to the event before the conjunctive miss the timing.
On the other hand, the conjunctives "
and", "
and if you do" and "
also" indicate that both the parts before and after it are done simultaneously. Therefore, no effect is taking place after it.
You can use Subterror Guru & Subterror Fiendess to trigger Subterror Behemoth Umastryx's effect.
Other trap card you can use is Ice Dragon's Prison.
https://youtu.be/7l4tSsSC7DU