Diablo in Normal difficult is the toughest monster the fishymancer faces in the whole game (Normal + Nightmare + Hell). Here's how my fishymancer defeated Diablo in Normal:
Before I triggered the last seal that summoned Diablo, I collected a lot of health and mana potions. I dropped the potions all around the center platform where Diablo would appear later.
I also opened a Town Portal near the platform for emergency purpose. Then I triggered the last seal and came back.
Optional: I could summon up to three Skeleton Magi. I summoned three cold magi that would chill/slow Diablo.
When Diablo appeared, I approached the center platform slowly and carefully. I moved toward the platform -- but not too near so Diablo would not showed up on my screen.
I moved close enough for the tip of the platform to appear on the edge/corner of my screen. Then I cast Clay Golem at the platform. The Clay Golem rushed toward Diablo. Then I quickly moved my necro and his army toward Diablo (but not too close.) The Clay Golem was keeping Diablo busy.
I decrepified Diablo and repeated and repeated. Decrep was very important. I did not wait until Decrep expire before recasting it. Low-level Decrep lasts for about 4 seconds, so I timed the curse and recast it: one... two... three.. DECREP! one... two... three... DECREP! ...
I kept moving my necro because Diablo could cast Bone Prison on a stationary hero.
I watched for the health of my Clay Golem. As soon as the golem's health was half empty, I immediately summoned a new one right beside Diablo. That way, Diablo would not shift his attention from my golem to my merc or skeletons. Diablo cast fire rings much less frequently when my golem was keeping him busy.
I watched my merc's health as well. As soon as his health is half gone, I gave him a red potion. I ran out of red/blue potion in my belt and inventory -- so I picked up the potions that I had planted around the platform earlier.
When I ran out of red or blue potions, I used the Town Portal to exit immediately. I had to exit and return a couple times to defeat Diablo for the first time. (I did not run out of potions on the groud; I just needed to catch a breath.)
The first Diablo was the toughest battle for my fishymancer. However, once that's over with, it would be a smooth and easy journey for the fishymancer all the way to the end of Hell.
P.S. A level-24 is probably too low to be fighting Diablo . My necromancer was near level 40 (in a 8-player game) when he fought Diablo, and he had troubles.
This will be by far the hardest battle. Though i also struggle with duriel if I try him without decrep. My usual tactic was to decrep and make sure to recast my gumby when he got low on life. When your merc dies (some damage reduction and life leech can help here) head back to town and take a tp to get a new army. Do this 4 or 5 times, and you should be able to clear it. It's certainly tedious. But keep it up, and you'll make it. If it doesn't offend your sensibilities, and you are playing online, you can always ask for some help. There will almost always be someone hanging out in the chat channel that will lend a hand.