Long ago, I wrote about the problems of fighting big monsters. Part of this came from watching my housemate at the time play through Shadow of the Colossus. In that game, you have to climb giant monsters in order to attack them at their (few) vulnerable points.
D&D, with its recent emphasis on a two-dimensional grid, has a huge problem accounting for these sorts of tactics. Instead, it seems to assume that you are always attacking giants in their shins. Or something.
In Destined, I think I’ve figured out a way to handle this. In general, I’m using the zone system that is found in FATE. Basically, a zone is a roughly-room-sized area. Normally, you can attack anything in your zone in melee combat. Ranged weapons can usually attack things a couple of zones away (depending on the weapon type). The borders between zones may or may not be difficult to cross. If two zones are connected by an open door, there’s no problem (unless someone is blocking it). If there is a wall between the two zones, however, there’s a difficulty that must be overcome to cross it in combat.
So, what I’m doing is giving big monsters their own zones. I’m still working out how, exactly, this will work, but the basics are simple. That giant? His back is a zone. It is non-trivial to get onto this zone, but if you do, the giant might be limited in how he can attack you. Maybe the giant’s armor is weaker there, too…