I did it with a 115 ranger and a silver 110 shm. You do NOT need to charm the defenders etc.
Once fight starts Shaman pet tanks Triune god and it is his only mission in this event.
Ranger agroes all defenders with summers and wait until both relics reach 60% of their hp, then the shards spawns.
Once both shards are up (never before) you can dps relics with summers spell to dont let them spawn avatars (at 80% of their life Avatars spawn), keep relics around 40% hp.
Once relics are at 40% or so, you can dps shards with bow + the 3 arrow spells (market shot, focused blizzard of arrows and composite fusillade) the shards will die with one set of arrow spells before they regen to 100% (shard regen to 100% every 6 seconds, use bow disc if your ranger is not enough powerful but I didnt need it).
Do the same with next shard. If you need it, cast summers to relics to keep them under 60% hp, it is important do not let them reach 80% or avatars will join to the party.
When both shards are dead, relics wont regen anymore and next set of relics will become active after a few seconds.
Repeat it with all rounds of relics (4 in total) and it is done.
You'll get the achievement at the end of event when chest spawn.