I have to disagree with the complaints about merfolk. Having a +4 polearm apt means you get to use the biggest polearm you can find pretty easily. I notice you guys keep talking about demon tridents, but the 2 handers are significantly more damaging, and if you're having a tough time killing enemies fast enough, that might be something to consider.
Secondly, they have a good dodging apt (+2 or +3, I don't remember). That means they have really good defense most of the time, as long as you train some dodging. And although they have a negative armor apt, later on there's enough xp to train it a bit. You probably don't want to wear gold dragon armor on a merfolk or anything, but you don't have to stay in leather either.
Another strong point is that in the water, they have incredible move speed (0.6 delay) and get a substantial boost to EV. If you can fight from the water as a merfolk, do it. Being able to cross water without flying is also nice, but not really a gamechanger.
Gladiator is a fantastic match for merfolks, start off training dodge and polearms, and just use the most damaging polearm you can find. Oka or trog are the obvious choices, but Ru can be great, as well as most any other god (probably not veh, kiku, or sif though)
Another great choice is skald. You get quick access to regeneration (merfolk have a good charms apt too), and infusion is better than nothing early on. Spectral shroud is surprisingly useful, it's always a good move to cast it before a fight if possible. Spectral weapon with a polearm is just ridiculous, you can stand in a corridor and have the spectral weapon behind you, almost doubling your damage output for just 4 mp.
Merfolk's biggest weakness is probably HP. They have a neutral HP apt, but they do have +1 fighting apt, which helps. So they're not exactly weak in this area, but compared to some of the more fighty races, their HP pool feels a bit low. Still, they're better off than elves.
As a quick aside, polearms are roughly equal to axes in terms of damage/xp invested, but you get reaching instead of cleaving. If you take the first free hit you get against most enemies due to reaching, polearms actually are the most damaging weapon for short fights. The longer the fight goes on, the less significant that first free hit is, of course.
As another quick aside, if you're looking for a different play experience(and possibly a pretty easy win) try a statue form troll. Start off as a troll TM or EE, then basically ignore your background and play as an unarmed troll fighter until a book with statue form shows up(hopefully it will). Then, learn statue form, and train earth and transmutations until you can cast it. Then, laugh in an evil voice and proceed to stomp the rest of the game. I've won two 15 rune statue trolls, which is the same number of trolls I've had who can cast statue form. Seriously, you can basically tab the rest of the game down once you can cast statue form as a troll.