If you’re playing a Two-Headed Giant game and a creature is attacking your teammate, you won’t be able to cast Qasali Ambusher this way.

Casting Qasali Ambusher as described in its second ability still uses the stack. The spell can be countered.

If you want to cast Qasali Ambusher by using its second ability, you don’t necessarily have to do it during the declare attackers step. You can do it later in combat (including all the way up through the end of combat step) as long as there’s still a creature attacking you. Of course, if you cast Qasali Ambusher later than the declare attackers step, it won’t be able to block.

For you to cast Qasali Ambusher as described in its second ability, a creature has to be attacking *you*. If creatures are attacking only planeswalkers, you won’t be able to cast Qasali Ambusher this way.

Qasali Ambusher’s second ability is looking for lands with the subtypes Forest and Plains. Those lands don’t need to be named Forest and Plains. It’s okay if both subtypes are from the same land (such as Savannah or Temple Garden).