I don't think you want to use insect plague (lvl 5 druid spell, 1 dmg per round, prevents spellcasting). Try Creeping Doom instead.
Don't even bother with missiles - DD are 90% resistant. If you have slashing-resistance items, load up one character with those items (and sing the War Chant song). If your F/D has hit druid lvl 15, the water elemental shapechange is highly resistant to slashing damage. If you were lucky enough to get the Bone Marrow Belt (random drop in the Hand) and have a lvl 15 druid, your troubles with drowned dead are over (100% slashing resistance).
Definitely use summons. Try to draw as few as possible at a time so you can make your summons last longer. If you can hold the DD for a short time in one place with summons and/or disablers, layer the area with spells. Spike Growth and Spike Stones from your druid are effective.
Also, make sure you buy the Three White Doves mace from the Waukeen cleric in Lonelywood. A chance to utterly destroy tough undead with a single hit is no small potatoes. You can set up your strategy to maximize your opportunities to hit and minimize your chances of being hit if you only attack DD that your summons have attacked first.
Here is something that might be of interest to you. It's essentially a walkthrough/strategy guide I wrote when doing IWD/HoW/TotL on HoF for the first time. There's a post about Burial Isle about a quarter of the way down the page. Watch out for spoilers.