I have always played with a full party of eight, either by creating each member myself from the start or by recruiting NPCs as soon as possible, and have found that such large groups never reach level 20 by the end of the game naturally. However, in the days before I heard about Temple+ and still played with Co8 alone, it was at least feasible to attain, albeit with copious amounts of grinding in random encounters. With Temple+ installed, I find that once a character reaches their 17th level, they cease to gain experience from encounters almost entirely. As far as I know, the only enemies you can run into while wandering the map that still yield anything at that point are ogre chiefs and nightwalkers, and they only ever seem to appear singly among groups of lesser monsters. I suppose it is theoretically possible to grind these particular enemies to reach higher levels, but considering their rarity and that they're only encountered one at a time, it would be a pretty dreadful slog to go through. It is of course not necessary to have 20th level characters in order to complete the game, even the Demons and Demigods quest, but I am curious nonetheless if there's any more practical method of gaining the necessary experience.
You can use the DM console to port in most enemies. Careful of clashing factions that can mean that your opponents don't end up fighting each other, though.
You can also use the console to set your EXP totals to whatever you want, as well as to gain arbitrary EXP amounts. I played locally with a friend with a team of 2 main characters, and we reached about level 17 by the end of Co8's new content, though we crafted thousands of EXP worth of items. By that point, we were able to spam horrid wilting and holy word to kill most things, and use haste to conserve spells and just attack. Hickory Branch and Hommlet Defense were my favorite part since we cleared them without resting. I suspect if you're soloing the game and you level strategically (banking experience and only leveling once you're 1 EXP short of leveling twice), you'll reach a higher level overall, though grinding may still be involved.
I think water node spawns ice tyrants which provide nice xp. Also you can turn on level squatting in Temple+ configuration (though it might be a bit late for you I guess...). I haven't checked but maybe wielding alignment opposed magic item lowers your level and nets you more xp?
I've stuck with six-member parties to enable all the quests, personally. Never had a problem reaching level 20.
