Armor, not weapons. I don't want complicated TL;DR posts. You can have two different parts, only two defenses per part like any other armor. This still lets you get all four, net. Keep it balanced, like trading off one status ressist for another. If you look at valkyrie/fallen, you can tell that a medium monster weakness is equal to low ASI. Low ASI is technicaly a medium level bonus, since it applys to guns and swords. CTR or damage would be high level. No more than one medium monster penalty per peice, so you cant make unkillable LD gear that gives max all monster damage penalty or something. No more than medium attack speed decrease per peice since bombers can work around it.

I'd like a tweaked version of the chaos cowl;
Normal defense increased to the level of the arcane salamander mask (+2 or 1?)
Elemental defense increased to the level of the arcane salamander mask (+2 or 1?)
No more posin weakness (+2)
Max fire weakness (-2)
Max freeze weakness (-2)
Max shock weakness (-2)
Max curse weakness (-2)
Medium feind damage penalty (-2)
Low damage bonus (+3)
Low movement speed increase (+3 or 1?)

Net -10 and +12 or +8

So it's either overpowered or underpowered by about one medium UV.

Would be nice for void levels. I couldn't tell how much of an increase it was to get arcane level defense, each increase could have been a low or a medium. Wasn't sure how much of an increase movement speed was either. I tried comparing mercurial helm to royal jelly helm, and their the same except for changed status ressistance and the movement speed difference.