Below are the different strategies against King Phillip.
King Phillip's 'main' spearman attacks should not prove a threat if a decent number of archers are used on top of walls and towers. They may be a little harder to kill than, for example, the Rat's spearmen, however, as they have a 25% combat bonus because of King Phillip's use of positive fear factor.
Phillip's knights are more of a problem. These are sent frequently by Phillip in groups of up to nine. They target quarries, iron mines, woodcutters' huts and farms, so a few good sized groups of macemen amongst these buildings on aggressive stance should help protect them. Some crossbowmen should also support them to take down the heavily armoured knights from afar. If you can enclose all of these buildings behind a wall, do so, as Phillip's knights, like those of the Marshal and Emperor Frederick, will only target exposed buildings. Make sure to use some large towers with ballistae along castle walls, as Phillip likes to roll forward catapults frequently if he has a lot of gold.
Repeatedly destroying his iron mines long before besieging him is a good idea to cripple Phillip's economy and stop him producing knights. Care must be taken, however as his knights may guard his iron mines.
A good number of horse archers (60 should be enough if they are constantly moving in front of his castle) and about 20 assassins are all that are needed to take out King Phillip (unless he is doing particularly well). The horse archers should make short work of his archers, spearmen and tower ballistae. If he charges you with his knights this may a problem, but if you move your horse archers way (but still in range) when the knights charge and keep moving them away as they try to engage, they should lose health rapidly due to the arrows. Once all his forces are dead, send the assassins to climb the back gatehouse and then send them to kill the lord.