This massive creature is unlikely to attack the PCs directly, but the PCs can still be hurt by its activities. Each round the giant is active, it causes damage in one area of the city. This damage might be by throwing things, using a weapon, kicking, etc. When targeting such a large area, the giant always hits, and does damage based on the table below (insert table, which should include secondary effects to characters in the area of effect).
The giant has 250 hit points. On any given round, city defences (archers, catapults, etc.) whittle away some of these points, but as the giant damages the city they are less effective. On round 1, the city defences do 2d10 damage. On round 5, this is reduced to 2d6. On round 10, this is reduced to 2d4. On round 15, 1d6. On round 20, 1d4. On round 25+, no further damage. The amount of damage done by city defence may also be affected by the giant's attacks, as shown on the table.
Treat the giant as AC 5 for the purpose of PC attacks. If any PC attack causes the giant serious inconvenience, or deals more than 10 hit points of damage, there is a 1 in 6 chance that the giant targets that PC specifically. This is cumulative; the second time, the giant has a 2 in 6 chance of targeting the PC. This chance resets if the giant successfully strikes a PC. The giant has -6 initiative, and attacks using 1d24 with a mere +4 bonus -- it is actually difficult for him to target creatures so small! Hits cause 2d12 damage, and cause a critical on a natural 20-24 (G/d12). The giant always succeeds in Fortitude and Will saves, but always fails Reflex saves.
The giant needs to demolish six city blocks to reach the palace. It can cross a demolished city block in a single round if it makes no attacks. Once at the palace, it takes the giant 2d3 destructive rounds to do enough damage to find the princess. It then backtracks the way it came, only attacking when seriously inconvenienced or when it has taken 10+ hit points of damage on a single hit.
Once it reaches the city edge, it moves at full speed to deliver the princess to the enchanter. Only horses at a gallop, swiftly flying creatures, or similar have a chance of keeping pace with him.
Cathbad the Meek (herbalist Wizard 1): AC 9; 4 hp; S 7, A 7, St 10, P 17, I 13, L 8; Neutral; Club, herbs, 50' rope, 50 cp; -1 to melee attack rolls. Hideous scar.