Going back to tabletop, I bring two artifacts from Lamentations of the Flame Princess:

First is the Monolith (from The Monolith From Beyond Space and Time). It allows travel across times and universes. It can be made to not exist, but given a co-operative character, this is a defensive feature more than a weakness. Pretty hard for other universes to destroy something that doesn't exist.

Second is the Book (from The God That Crawls). It is a spellbook that, once assembled, destroys all information other than itself. All. Information. Starting with other writing, ending with natural constants. The Monolith would allow a relatively safe and easy way to take its pages, one by one, to a target universe. Once assembled, it would make a potent weapon against them. Notably, nearly all sci-fi universes would be suspectible, because if they can't neutralize it near-instantly, all high-tech solutions for locating it are ruined. Sadly, the Book is destructible, though it renews itself (on the pages of other books, which must be recollected and reassembled).