without debuffs:

with Weaken:

with Division:

with Division and Weaken:

as you can see Weaken doesnt change the dmg while Division increases it by a lot. My guess would be that Weaken lowers the defence and Division simply increases the dmg done to the target.
Edit: On the screens the dmg with weaken seems to be a little higher, but thats only cause of the damage-range. I tried it multiple times and it ended up being the same, sometimes a little bit higher and sometimes lower.