>>528039227
>>528040768
>>528040180
I'm a little late to the party here so I don't know if answers are still needed, but you can get the extra event commu by just unlocking the five new say "Halo" L'AntxSHHis commus. You can use any idol so long as they're from one of the two units and all five are shared between them. I did one run with Sakuya and got 'em all by the end, so it's not too hard.
As for actually playing say "Halo", there's basic strategy at the bottom on the scenario page on the SC wiki.
https://shinycolors.wiki/wiki/Say_%22Halo%22#General_Strategy