Hardest enemy to kill, hmm, I don't know, probably the silver dragon (Red (firkagg)was fairly easy as I had an immunity to fire, shadow was easy as I had immunity to level drain (my main character was a undead hunter
) and the black dragon was quite hard, but I had lots of very high level spells by then).
Adamantium Golems are quite hard, but get stuck in small spaces really easily
.
Also, I found groups of medium strength enemies in small spaces quite hard (like enemy groups in small room etc) especially when the room has no exit except for an auto-save door and the creatures won't follow you
.
And finaly I found the silver sword blade Githyanki and group quite hard (even with Immunity to +1 weapons or less on my main character, as they targeted the rest of my party who got mashed
).
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Nicodemus Deathslayer, Paladin of the Order of the Holy Flame