Thanks for pointing the right script, Tempered.
And thanks for the idea, GetAssista.
Yes, I should make my own "shield_item_set_banner" script. I think I found the problem. Normal battle automatically link troops spawned with their party, but not for my "simulated" battle. So, for my agents,
(agent_get_party_id, ":agent_party", ":agent_no"),
always give me ":agent_party" = -1.
EDIT :
It works now, thank guys.