Kalagru is an Outsider, so
Banishing Weapons are ideal. He is healed by FIre, has
very strong resistance to Shock, and
strong resistance to all other elements; along with his high Armour this positions the
Adamanite set (pure physical) as likely the best weapon candidates with Light, Hand and 2H choices dependent on build.
Unfortunately spell-casters have no Adamanite options, so Clerics should look to
Maul of Terror (there are no strong Light or Hand options for Clerics for this battle).
Wizards should be looking primarily at spells... but for a weapon
Staff of Impact or possibly
Staff of the Traveller are the options.