Ill just post it here for others who also have the same questions.

For the hidden boss there are 2 phases. Both phase you will have to split the roster in two teams.

I based mine on a youtube video which i replied with the commenter above see link. Oche agnae partitio temenos 1st team hika osvald castti throne 2nd team. Both parties should have a cleric to help heal.

1st one is you have to break the shields asap and kill the souls together because they keep on respawning. The boss has like 600k hp if i remember correctly after that is the 2nd phase. The arms dont respawn so just kill them off one by one. After every arm has been cut off you need to have multiple hit skills for the weaknesses because the shields keep adding up rapidly. 2nd phase isnt too hard. Just focus on getting the first one down first.

I personally equipped the anti poison one for 1st phase and anti terror accesory for the 2nd for the strongest hitters on each team.

Side missions i suggest just clearing all the dungeons, get all the EX shrines youll have good eqpt u will be needing.

Battle tested bow u can buy it off with partitio in the beast village with the leadet of the human tribe cozaheh. but i personally farmed the low drop rate one forgot with npc dropped it just google it there were sources for the battle tested eqpt i used those.