TIP: Battle effects can already be toned down in Character Options.

This is located in the weirdest of places.

Character Configuration
[ Control ] option button (first one, selected by default when you open this window)
Character tab

Battle Effects Settings

Own

• Show All
• Show Limited
• Show None

Party

• Show All
• Show Limited
• Show None

Others

• Show All
• Show Limited
• Show None

I currently have Others to Show Limited and it seems to help slightly with the phasing in/out problem on mobs being attacked by hordes of players. It still does happen occasionally though. Target-lock on the phasing mob also seems to help slightly.