I like the idea of making combat more interesting, but I dislike the idea most recently supported by Schmoe.. {not picking on ya

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My reason for disliking that scheme is that you are introducing a completely new set of mechanics for a situation that is much like ones already covered. I think that by adding special attack {see my previous post} and the use of 'soft spots', you can raise the entertainment of the encounter while still using mostly core rules.
This has the advantage that each Collosus will still be distinctly different, rather than a trample monster, provide more options for both the DM and the Players...and still reflect some of the derring do that is preferable.
For the 'soft spot', perhaps instead of actually mapping out spots, use a 'stunt' like option where a skill check can get the PC to within striking distance.
Something like...
Move action to climb/jump to a 'soft spot'
Move action to cling to a 'soft spot'
Attacking on a soft spot is equivilent to flanking and negates some of the beasts natural defenses.
You could have a generic table of DCs, something like:
DC 15 to reach a soft spot, gaining the benefits of flanking
> This DC is modified by how critical the spot is:
+ 5, ignore the beasts natural Damage Resistance
+ 5 increase your weaons threat range by 1
DC 20 to cling to a soft spot, limits attacks to one-handed weapons
This keeps us DM's with less time to prep with a simple mechanic that can be flavour texted out with the fancy moves.
====warning... the above is the rambling of a HR-aholic... DC, concept, and execution all possibly way off.... ========