How to balance this encounter

So, I’ve been playing with a party of six, and so far the fights have been pretty easy for them.

They’re level 3: Star Druid, Gloom Stalker, Samurai, Lore Bard, Celestial Warlock, and Giant Barbarian.

They’re currently exploring a crypt, and I’m planning a final encounter with a wight, two ghasts, and a hell hound , it’s meant to be a tough fight.

Their goal is to destroy a totem that awakened the dead and lay them to rest. I’m not sure how long it should take to destroy it, or how difficult it should be to reach, so the fight doesn’t end in a single round.