Was curious so I logged numbers from the first pest fights of 12-5 over 2 runs, since they only do normal attacks:
First Run
Everyone but Edelweiss gets 12% def buff
929 - Bipinna (Important)
478 - St Paulia (Junior)
Everyone but St. Paulia gets 12% def buff
1119 - Orchid (Important)
570 - Orchid (Junior)
979 - Canola (Important)
Everyone but Canola gets 12% def buff
914 - Edelweiss (Important)
449 - Bipinna (Junior)
Second Run
Everyone but St. Paulia gets 12% def buff
461 - Canola (Junior)
Everyone but St. Paulia gets 12% def buff
913 - Bipinna (Important)
474 - Canola (Junior)
Everyone but St. Paulia gets 12% def buff
430 - Bipinna (Junior)
Base Defense + Equipment Defense
Edelweiss - 1720 + 691 = 2411
Bipinna - 2220
Orchid - 1750 + 111 = 1861
Canola - 1576 + 619 = 2195
St. Paulia - 1480 + 690 = 2170
Enemy Attack Values
Important - 3125
Junior - 2138
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So it's more complicated than that (otherwise high defense would actually be really good) and there's a damage range or random factor. I know with enough defense you can end up taking only 1 damage from stuff though.
EDIT: Also, my Orchid has 550 less DEF than my Edelweiss (616 less DEF with Bipinna's buff), but is only taking 205 less damage from Important (less than a 20% damage decrease). St. Paulia has 5300 more health than Cepha Lanthera but 660 less DEF. With similar scaling, Cepha is going to go down 2-3 turns earlier, which is often a full extra battle earlier.