If the Arrow is going to request help to take down Ra's Al Ghul, the natural assumption would be that he calls up Grant Gustin's Flash. The Fastest Man Alive should be able to take down even a talented warrior like Ra's Al Ghul in no time at all and still have time to stop at Big Belly Burger before anyone in Central City realizes he's missing. But according to Stephen Amell, the Arrow get some help from someone he would definitely not want to be working with. Speaking with Variety, Amell stated, "I think [Arrow] comes back [to Starling City] a little more soft-spoken, a little humbled and forced to do things and work with people that he never would have considered working with before. I wouldn’t go so far as to say he has to make a deal with the devil, but pretty close.”
Bearing Amell's comments in mind, the two logical choices would be Manu Bennet's Deathstroke (who's confirmed to return in Season 3) and John Barrowman's Merlyn. Either one would be a worthy ally in a fight with Ra's. Hell, for someone of Ra's capabilities, the Arrow might want to consider bringing in both of these guys to fight Ra's and the League of Assassins.
This Wednesday will see the 12th episode of Arrow season 3 premiere and with the show's production team currently filming episode 3x17 "Suicidal Tendencies" there's just 6 more episodes to shoot before Arrow fulfills its 23 episode requisition. Things can change in those last 6 episodes but it sounds like Arrow has to make a deal with one of the bad guys to take down Ra's.
Arrow airs at 8PM, Wednesdays on The CW.