I am currently leveling a Repeater build, it's kind of odd and is actually an attempt to make use of Halfling Dragonmarks.
He is 8 Ranger/2 Fighter/1 Rogue, will end up 13/2/1.
Most people will tell you not to take Ranger with a xbow, but for this build there ends up being some very useful synergy:
1-Ranger feats free up a few for Dragonmarks
2-Ranger skill points let me keep up Rogue skills so I have some secondary usefulness
3-Rangers get +% cure enhancements to help with the DMs
4-A few very useful spells
5-With one feat (Weapon Finesse) I am a passable melee combatant
6-(One I don't think many thought of) Ram's Might adds to xbow damage!!!!!!
This build is working out spectacularly so far, including some elite runs, and some forays into Gianthold at starting at level 9.
(At level 9, I was unable to find the Cabal trap, but also only had regular Heroism and Search +7 goggles, at level 10 with Search +10 goggles and the one extra skill point, I can find it no problem on normal)
I imagine you could sub in the Great Crossbow for repeater without changing any other part of the build.
For those finding themselves falling behind in Elite, just follow the melees example, go with Puncturing.
Now, Wounding of Puncturing can be real rough to find, but Keen of Puncturing (ML:6) can be found relatively easy (bought mine of the AH for 2200pp), once Imp Crit is picked up, all kinds of special things can be tossed on the front.
My current combo (which is kind of ridiculous to watch) is a Cursespewing Light Repeater of Puncturing, coupled with Mummy Wrappings. It is fun to watch a mob come running at you, get cursed, hit you once, and go running away in fear.
This character is not going to be leading kill counts very often, but I am almost always in the second tier. Combine that with disarming all the traps and throwing the occasional Heal (<-Note capitalised, because it's the Heal spell), and you have an amazingly well rounded character with great survivability.
PS
I have noticed that on the repeater, Rapid Shot increases shots per minute more than Rapid Reload.