If you summon Sherra any time before Toxxulia engages you, you will prevent the "hard mode" encounter from occurring for the length of that instance. For a "hard mode" encounter, summon Sherra after Toxxulia has engaged your raid. Using the summoning crystal is a raid-wide update.
If you summon Sherra before engaging Toxxulia for a normal encounter, you will only have to defeat her minions, Toxxulia will fly away when Vuulan is pulled.
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