> My second party is now all level 14, except for Phaser, my Wizard, who is still level 13.
>
> Level 8 is way too difficult for them. I just went there four times and each time my entire party got killed on their first fight.
>
> Level 5 is way too easy for them. I just made a pool run to the 3 pools in the corner and I got less than 10000 experience per character. At this rate I will have to make 200-300 pool runs to advance the characters to a sufficient level to face level 8, let alone level 9.
>
> So what should I do? There's always levels 6 and 7 between levels 5 and 8, but what is there to do in them? Can you suggest some route on level 6 or level 7 that would result in much experience?

If you find level 5 easy, 6+7 should be a breeze. There aren't many pools on 6 but level 7 has more monsters. What about going through level 5 and keep battling the boss monster -- There are heaps of monsters between him and the stairs..... and the boss gives you loads more experience then the pools do. (As long as they aren't paralysed or worse dead....)
What type of characters do you have? Have you thought about retraining a couple of them? What kind of spells do your spellcasters have? Try going for agility rather than strength -- and lighter armour to give your fighters the chance to attack first on level 8.
Or you could try just raiding on level 8 -- fighting a couple of groups of monsters and then heading back to recuperate.