The Reaping Mauler is among the worst Prestige Classes ever written. It requires the useless feat Clever Wrestling, which requires you to be medium size. Becoming larger than medium size by whatever means is the key to successful grappling, though. So whenever you get an Enlarge Person spell from your trusty arcanist, you lose all benefits of the Prestige Class.
Not that said benefits are great. Among other things, it gives you Improved Grapple as a bonus feats some levels in, so you're 8th or 9th level or so when you get it. Do you really want to wait so long to get that crucial feat for a grapple-focused character? Nah. Also, the rest of the class features are quite sub-par. Forget about Reaping Mauler this instant. It actually makes you worse at grappling.
So, after the PrC-bashing, on to the useful stuff:
In a grapple, you deal unarmed damage. So upping your unarmed damage is the way to go. Monks do that nicely. However, two levels of Monk is enough for any Monk build. Get a Monk's Belt to increase unarmed damage more, as well as Improved Natural Attack/Superior Unarmed Strike, and, of course, size increases (Enlarge Person, Expansion, and/or a large sized race are nice and partly stack). Size increases not only increase your Str and unarmed damage, but also your grapple modifier, and let you grapple bigger foes.
Increasing damage and grapple modifier can also be done with a Cleric build - go with the usual Divine Power and Righteous Might, and get Girallon's Blessing for more arms, claw damage, and a rend attack.
The king of grappling damage is the Black Blood Cultist prestige class. These guys grow fangs and claws when in a rage, get Improved Grapple as a bonus feat early on, a little bit of damage reduction, and at 8th level get the excellent Savage Grapple ability: on a successful grapple check, you deal additional damage as if you had also hit with all your natural weapon attacks at once! This guy can easily deal 100+ damage/round in a grapple. The more limbs you have, the merrier.
Shapechanging is another possibility. A Druid or Wildshape Ranger can assume many deadly grappling forms - that is, forms with the Improved Grab and Constrict abilities.
A Fleshraker Dinosaur is an excellent form, for example: on a charge, you can make a full attack, with poison, that also trips your opponent and has the equivalent of Improved Grab stacked on. Bear forms are generally very, very strong and have Improved Grab. Snake-like forms often offer Improved Grab and Constrict.
If you go into the Master of Many Forms PrC, many more options open up. Just comb through the monster manuals and come up with the most lethal grappling monster you can find.
