There is a force ceiling 30' up (transparent). Ambient (daylight) illumination.
  The river is 10' deep (I made it deeper) in the middle and sloping up to 5' at the edges. It flows from SW to NE at a speed of 10'/round.
  The ground is dirt and/or sand down to 10' where it is sealed off by another wall of force.
  The stone structures are 15' high and made of stonemasonry (DC 20 climb check). The stone has hardness 8, 90 HP (per 10'x10' section) and the Break DC is 35.

  The bridges are wooden (hardness 5, 60 HP, Break DC 20).
Isigar starts in green, the giant in red. Go!

The druid materializes in the arena next to his stony companion.  He whispers something to Kaalk - though, he need not have taken such pains to keep his voice down, as nobody else in the area speaks Terran.

Initiative 11

The giant mutters something and checks the number of boulders in his sack. He seems eager to fight.

(The giant doesn't really prep anything. He has a club in one hand and a sack in the other)

Init: 13

The oddly-matched pair - the unassuming, unarmored traveller with a scroll in his hand, and the eight-foot-tall earth elemental in its spiky mail bearing an impressive spear - begin their preparations.

Prep Round 1:
  [Standard] Cast spell:  Call Lightning
  [Move] Keep casting

Prep Round 2:
  [Swift] Activate amulet (2 charges)
  [Standard] Cast Bull's Strength from scroll (3 Min) (share with Kaalk)
  [Move] Store scroll

Status:
  Initiative: 11
  AC (Touch AC): 18 (18)

  Damage Taken: 0
  Damage Dealt: 0


Effects:
Call Lightning: potential to call lightning (5 minutes, 5 bolts)
Bull's Strength: +4 Strength (30 rounds)
Primal Instinct: +5 to Initiative, Survival (pre-cast)
Heart of Water: Swim, breathe water, invocable freedom of movement (pre-cast)

Final Position: W13

Kaalk

Prep Round 1:
  [Standard] Look around

Prep Round 2:
  [Move] Ready weapon

Status:
  Initiative: 11
  AC (Touch AC): 21 (9)

  Damage Taken: 0
  Damage Dealt: 0


Effects:
Bull's Strength: +4 Strength (30 rounds) (shared from Isigar)
Primal Instinct: +5 to Initiative, Survival (pre-cast) (shared from Isigar)
Longstrider:  +10' movement (pre-cast) (from Isigar)

Final Position: W15

The giant stretches itself up to its full length and with a powerful heave of its muscular arms and shoulders pushes itself up on the stone structure. Boulder in hand the giant waits.

Round 1:
    [Standard] Ready action
    [Move] Grab and heave to top of structure (22) success
    [Free]


  Status:
    Initiative: 12
    AC (Touch AC): 20 (8)

    Damage Taken:
    Damage Dealt:

  Effects:


  Final Position: DE14/15, 15' up

To be able to climb up the wall and still have an action left over to ready, the hill giant needs to drop whatever he's holding (as a free action).  But this contradicts the descriptive text that the giant still has a boulder in hand when he gets to the top of the wall.