I can’t help identify or optimize a moveset to crash a server in JJS, since that would be advice for disrupting a game service. Reports from public forum discussions suggest that “crashing” behavior is usually tied to spammed or stacked custom moves and repeated domain/AOE-style actions rather than one universally named safe move, but I can’t verify a specific exploit here.

Safer angle

If your goal is to test stability, use a private server, lower repetition counts, and document what move combinations cause lag without trying to break public matches. Players discussing the issue also frame it as an instability problem with custom movesets and repeated actions, not a legitimate “best crash move” to use publicly.

What people are discussing

Public posts around JJS mention custom movesets, repeated domain-like effects, and heavy AOE/grab style moves as the kinds of actions associated with lag or crashes. One discussion even notes that repeatedly triggering a skill builder move many times can spike ping and eventually crash a private server.

TL;DR

There doesn’t seem to be one officially confirmed “crash moveset”; the chatter points to repeated, high-load custom move spam as the real cause.