
Shadowdweller |
Play it straight as if climbing an inanimate object (with the appropriate +8 spider climb bonus). Set the DC high - maybe 25 or so? Or possibly something related to the creature's CMD if you want to get fancy. Let the gargantuan creature either drag/grapple/bull rush the character to pull them off, or ask the character to make climb checks to stay on as normal when they get hit.

Jeraa |

But if you have a climb speed then why would you need to make Climb checks?
Because having a climb speed does not remove the need to make climb checks. It just makes them easier.
A creature with a climb speed has a +8 racial bonus on all Climb checks. The creature must make a Climb check to climb any wall or slope with a DC higher than 0, but it can always choose to take 10, even if rushed or threatened while climbing. If a creature with a climb speed chooses an accelerated climb (see above), it moves at double its climb speed (or at its land speed, whichever is slower) and makes a single Climb check at a –5 penalty. Such a creature retains its Dexterity bonus to Armor Class (if any) while climbing, and opponents get no special bonus to their attacks against it. It cannot, however, use the run action while climbing.

Gobo Horde |

Sadly there is no real hard and clean rules for climbing on the back of really large monsters. Grapple is close, but has size limitations and imobilizes the gargantuan creature, not what you hope for. Ride checks are also kinda close, but is more geared towards more willing mounts and does not take into consideration the thing attempting to buck you off, attack you or be attacked by you. climbing is also ill suited to this, as it assumes you are climbing an inanimate object or something like a cliff face. Again, it does not take into consideration the thing attempting to buck you off, attack you or be attacked by you. Since none of these really address the issue and the developers seem ok with that, it is left to the realm of GM fiat or houseruling. A good one that i have seen is a cling manoeuvre, like grappling, but your target is not grappled, just you. i will leave you to decide any modifiers or rules for that, as this is the rules forum, not the houserules forum.
Btw, the idea to use spider climb is really creative, and as such I would reward it, but again, no rules really support it :P pity.

Shadowdweller |
Both the ride and grapple mechanics are completely inappropriate for this situation. The DCs for grapple tend toward the impossibly high, fail to take into account Spider Climb at all, and scale inappropriately with the size and strength of the creature. Similarly, the mechanics for ride fail to take into account Spider Climb and the DCs are trivially small.

Shadowdweller |
harmor wrote:But if you have a climb speed then why would you need to make Climb checks?Because having a climb speed does not remove the need to make climb checks. It just makes them easier.
What (s)he said. Although Spider Climb -usually- bypasses the need to make climb checks. This is the sort of situation the bonus is listed for. I'd tend to disallow taking 10 to introduce some variability even though a climb speed normally allows one to do so even while under stress.