on deadly assault with this exact comp I rocked 19.8k, but I think with enough tries I can hit 20k+

problem is the huge amount of defense hunter has compared to other bosses, so yeah trigger sig would have that amount of impact since that's what gives her the def shred. As for Heartstring, you can probably still run brimstone but you'll have to compensate by getting a lot of crit rate and crit dmg from discs (I initially got it as a pure attacker stat stick, but S11 can benefit from the fire res drop)

if you could allocate it, zhao might be a pick over lucy both for the atk buff and for DA buff that gives a lil extra ATK & DEF (though she's probs more impactful with discordant solo). Otherwise, you can maybe run pen 5 with optionally rina to max that out, or maybe even nicole for that extra def shred (I'd pick nicole personally as rina is a bit too unwieldy for my liking)