If memories serve me right, HoW is designed for a party already finished the OC.
Of course, if you start the OC, you can jump to HoW before the point of no return. If you finish HoW this way, you will be teleport back to OC and continue. (And i did it twice, if you want to know.)

I did IWD on a different PC, so I can't import the winning party.

I don't want to give up, but i just can't beat the masses of attackers in the dungeon, then i will have the end sequence too (which i have got to, but went back to the Dungeon to earn more points).

I've also set it to "easy" but they swarm all over me.

Is it the dungeon in Burial Isle?
It is really hard if not well-prepared, and I end up spamming Web with hit-and-run.
Their HP won't regenerate even you leave the dungeon. You can take advantage of it.

I ran a party of 5x f/c/m and 1 f/t/m
fcm's played like fighters for the first couple of levels using healing spells between battles and wizard spell slots for identify
If you don't mind rerolling quite a bit for high stats, you can finish the main game on highest difficulty with this party. (maybe not the last boss though) high wis for clerics doesn't matter much
sleep and grease are great spells at low levels and clerics should never prepare any lvl3 spells except animate dead. Just use your summons to draw attacks and soak up damage while your chars wipe out the enemies.