IIf you want to have it while really playing, the easiest way is to pick sniper at the beginning of the round and wait for a scout, soldier or demoman to go in the rafters above the central capture point. Picking demoman with scottish resistance and having sticky bombs on each raft is not a bad idea.
If you want to go in the raft and wait for ennemies, you can pick : scout, soldier and demoman!
GL HF!