Where can I find a firing squad (for Time Takes Its Toll and A Forceful Rebuke)?

I got both of these achievements in Act II. The desert is full of wasps that shoot littler wasps, and these littler wasps count as projectiles.

Simply find a group of wasps, cast Slow Time, and stand in the center. Since each wasp fires volleys of 4, you should get the "slow 20 projectiles" in a matter of seconds. Afterwards, simply use wave of force to send them all flying for the second one.

Act IV, End Boss. When you are fighting him the third time, again in the real world, he has an ability where he casts a stream of "fire / lightning at you." That stream isn't a beam, but many single projectiles.

I got the Time takes its toll achievement there. Don't know about forceful rebuke, since I didn't try to fling these projectiles back.

Or you can try to find a big group of these Moonclan Javeline Throwers in the first act. With enough hp/armor you might be able to pull enough of them to get these achievements.