Wow! Also, confused (Destiny)
sorry, should've posted the reddit thread as well.
Once the arc-soul gets the damage buff, it keeps it as long as you can reset the arc-soul timer before it runs out.
Complete thread:
- Someone beat Calus using ONLY Arc-Soul - 
Schedonnardus, 
2017-11-20, 08:07 
- Wow! Also, confused - 
Beorn, 
2017-11-20, 11:14
- Wow! Also, confused - Schedonnardus, 2017-11-20, 11:19
 
 - Not sure what's more impressive - Kahzgul, 2017-11-20, 14:05
 
 - Wow! Also, confused - 
Beorn, 
2017-11-20, 11:14