How do you get Prismana Fulmintis?

That means money rather than grinding. The pass is the Companion Handbook, also called the Companion Guide, and it ships in two editions. The standard edition gives you the free top track only. The optional Advanced edition costs $10 and unlocks both tracks, and the unique Aniimo form is on the paid side. The broader structure is a seasonal pass with a free track and two paid tiers, so check which tier you are actually buying before you pay.

Buying in is not the whole job. Rewards unlock gradually as you earn battle pass experience, so the Aniimo sits behind a season's worth of play as well as the purchase. The same track carries cosmetics and journey-assisting items, which is the usual reason to buy the Advanced edition even if the Aniimo is what you came for.

The exclusivity is temporary, not permanent. It is tied to this pass rather than locked away forever, though nothing states what the route back is or when it opens. In the closed beta the advanced battle pass carried this Aniimo under the name Prismana Fulmentus, so the spelling you see may shift between builds.

Getting Prismana Aniimo without the pass

Prismana as a type is not entirely gated behind the Handbook. Nurturing is a way to obtain Prismana Aniimo, and how central it is has not settled: it is described both as the primary method and as an alternative to other routes. Either way it applies to Prismana Aniimo in general, not to Fulmintis specifically, which stays on the paid track for now.

Not settled: The exclusivity is described as temporary, but nothing specifies whether Fulmintis returns in a later season, becomes nurturable, or arrives by some other route once the pass ends.