It's on the large event poster right below the game:

"EP Gained in the 1st Phase, will also increase the encounter rate of Super Elite Guards during Phase 2!"

and a bit lower:

"In addition to raising your SED, SECs will now increase the encounter rate of Super Elite Guards during Phase 2!"

However, there is no in-game banner for reference anymore, and seeing how a friend who doesn't have any Slayer Cards and had a much lower score during Phase 1 seems to spawn them about as much as I do, I can only conclude that the encounter rate increase is either non-existent or very marginal.