How to get minions to not wander too far off from boss?

So I have a mob (reaper) and when you kill its minon (shadow) it goes into phase 2. The only way to change the phase is when the minion dies target all reapers in a 100 block radius and give them the tag phase 2. Except this can lead to 2 errors.

1. The shadow wandered 100 blocks off and once you kill it the reaper is stuck in phase 1 because the shadow was too far off to trigger the phase switch and the boss is broken

2. Someone spawned 2 reapers close together, reaper A and reaper B. If...

How to get minions to not wander too far off from boss?