The above describes an "normal" mode encounter in which you get to complete the zone without having to defeat Toxxulia. Summoning Sherra before engaging Toxxulia means you will only have to defeat her minions, since Toxxulia will fly away when Vuulan is pulled.
Despite the quest dialogue which implies that Sherra should not be summoned until Toxxulia is engaged, actually doing so will mean you have to repeat the zone, since summoning Sherra will be followed by the "kill Tuluun, kill Zaos, kill Vuulan" steps"; and you cannot engage Toxxulia until after these are killed. This means you cannot combine completion of this quest with a "hard mode" encounter in which you get to fight Toxxulia.
For a "hard mode" encounter, just enter the zone and stroll up to the mobs. You can summon Sherra after Toxxulia has engaged your raid if you like (using the summoning crystal is a raid-wide update). Unfortunately, what the quest updates to is "kill Tuluun, kill Zaos", and you will have to wait 3 days to reset.
