It may be advisable to prepare multiple holes at one time if rented equipment is used. An alternative technique is digging underground diversion channels from hole to hole utilizing gravity to divert water downward. If your problem is heavy clay topsoil causing drainage problems, consider repetitive amendments of sand and coarse organic material. Over the years this will promote better surface drainage. Planting on slopes presents a drainage problem of sorts.
Slopes discourage water from getting the the palm's roots.
Solutions include slow emission of drip irrigation, adequate watering wells, or mulching to slow down the downward movement of applied water.