Ok, first of all, this team probably won't make it all-too-far in it's current state (Not insulting, just stating the facts) - a lot of these Pokemon you are choosing are not exactly worth all that much (most are UU or NU in gen 4, and RU or NU in gen 5)
First of all, While Absol may have a nice attack stat, It doesn't have the speed or versatility to be able to function well, the Common Absol set is SD/Sucker Punch, because that's all Absol has going for it.
I suggest maybe substituting for Weavile, Weavile is faster, has almost the same amount of attack (not sure the exacts) and is much more versatile in terms of moves to use...
Next up is Nidoking - do you know about Nidoking's insane new ability Sheer Force? It gets it in the dream world, and a Modest LO Nidoking sporting The three Elemental Beams and Earth Power can really pack a punch...
...sadly, it's not all-too fast, and is best combined with a Shell Smash or Agility Baton Passer...
Now Shaymin - Shaymin Doesn't have Serene Grace!!! Shaymin-Sky Forme Does, but that's a Banned Pokemon in competetive play...
...If you are set on using Shaymin-S, because you are playing some crazy tier where it isn't banned (BTW I follow Smogon Tiers religiously) then the preferential set is:
Shaymin-S @ Choice Scarf
-Hidden Power Ice
...there are a few other sets, but like I said, Shaymin-S is banned... If you want a Pokemon that functions the same (and I'll hate you if you want such) then Jirachi or Togekiss are the Way to go: Iron Head/Air Slash combined with Thunder Wave or Body slam makes for a fkn HAXy demise...
Quagsire is useless in Gen 5 - Go for Swampert or Gastrodon instead - Swampert has much better stats, and is a better wall BY FAR, while Gastrodon has Storm Drain - allowing for you to switch in on Water-type moves, and proceed to sweep the opponent with a Powerful Surf/Earth Power/Ice Beam combo...
Finally, Mantyke... Why on earth would you want it??? If you are looking for a special wall, Blissey and Chancey are your best bets, they are the undisputed Best Special walls...
and if you don't like them, Try Reuniclus... Reuniclus is not only a nice Special wall with Calm mind, but with Access to Psyshock and the unresisted Shadow Ball + F-blast/HP Fighting (F-blast has crap accuracy) - it can actually fight back!!!
In conclusion, this team does need some work, and if you want any more help on it, feel free to VM or PM me - I'll be happy to help out...
also, a couple of Links to help you build your teams:
the BW dex is not yet complete, but hey, DP works! (I know, I'm DP xD)
if you are really looking for BW analysis's though, you can always check smogon's Forums, it's got constantly updating analysis of all gen 5 pokes - but really, Being Creative every now and then is good too. How else would I have used Gigalith (an NU Poke at best) in competetive OU play to such success? TR + Gravity makes for a nasty EdgeQuake...