If you don't have the 'shield of balduran' then cast four web spells into the area (web spells are not treated as offensive - so they won't attack you at that point), then as many cloudkills into the same area as you can.
Just to be safe,(Beholders have some magic resistence), I usely get Jaheira to cast entangle spells at the same time my mage is casting the cloudkill - that usually stops anyone from getting into range to attack you.
But of course, you really should buy the 'shield of balduran'...
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