So for the most part ATOMS are pretty straightforward. However whether it be an individual event or alliance stratigically opening your might can make all the difference.
First off, if you are only going to be around for the beginning or mid points of an ATOM event then your best bet is to open everything (unless told otherwise by a crown).
However if you are going to be around at the beginning maybe somewhere in the middle AND at the end, then some strategy can come into play.
To start off, I would pop ALL T1, T2 and T3 items EXCEPT for the 10K PACKS. You could also open your T4 (most people don’t tend to have a lot boxed) as well as your lower might tactical (MASTER RANGERS and PIKEMEN). Save all T3 10k PACKS, T5 and high might tactical for the very end (ie, last 5 minutes or so). This will ensure in the case of an individual event that you have a nice cushion of might to add at the end if needed and in the case of alliance events it gives the alliance a nice cushion of might as a just in case.
As always, if word comes down from any crowns that you need to play differently, follow those directions.