i'm level 38 almost 39 trying to do these 80 missions for this event but also 16-4 and feel stuck. i just started the game so my nonomi is only on day 4 i can't even bond 15 her yet.

which unit can i dump all sorts of stuff into to carry me on piercing against heavy units?

nonomi
momoi
akane
pina
hasumi
seia
meru
junko
kotori
tsubaki(i use yuuka as a tank)

new year serika
ayane swimsuit (these carries and does crazy damage should i upgrade her equipment?)

am i too focused on piercing for heavy types? what if i bond 15 serika can she mow through these level 80 heavy units as 39ish