While you don't have to take a full 6-man party, the game was definitely not designed to be solo or duo friendly. Many of the encounters were designed with the expectation that you'll have multiple party members who can split their attention between several foes and/or assist an incapacitated party member.

If you're very familiar with Infinity Engine games, you could probably manage with 4 or 5 well-built characters. I've heard of some people soloing the game, but you really need to know what you're doing, be very familiar with each encounter so you can prepare the appropriate defenses ahead of time, min/max to the extreme, and take advantage of cheese tactics and exploits whenever possible.

The game gets considerably more difficult with smaller parties since higher level characters get less combat XP so small parties don't level much faster and get 0 XP for fighting quite often.

Soloing is doable nevertheless, any class works. Pure class cleric and pure class sorcerer are the best options followed by pure druid. A duo consisting of cleric and sorcerer will have more options available.

Be aware that stats do matter, items only add to the values and don't set to fixed numbers, the casting stat and the physical stats are far more important than the remaining two mental stats which can be dumped to the minimum.