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.