I decided to do some more tests. It works with whatever damage weapon you use. Here is this Navrey normal and with reduced resists, depending on weapon. Consecrate has no effect on which resist, its the weapon. I tried the 3 weapon damages I had on me. (Not sure what would happen if you have a mixed damage weapon)
I just clicked the Onslaught Icon and didn't do many specials. Many times I get the prep message (You ready an Onslaught) and no result message (You deliver an Onslaught), So I assume it missed the hit and you have to activate it again. The only time the resist would change was when I got the result message.
It works with any damage type, but its best to use the lowest resist of the Mob of course. But, even if you have the "wrong" weapon, it reduces that resist.
This is an ability that benefits all players hitting the target, if they are using the same damage as you are.
I don't know if it would stack if multiple swordsmen used Onslaught with the same weapon damage. (my guess is it would for the time) I'm sure it would stack with Discord or Rune Corruption or another pet special that reduces a resist.
If you Discord the target, Onslaught will reduce the resist more and add even more damage.
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It is a set number reduction. It reduces the resist by 16. With 120 Swords and 115 Tactics.