The glossary entry for Phasing is rather sparse, and doesn't seem to address any specific issues that could arise while phasing. Phasing: While a creature is phasing, it ignores difficult terrain, ...
Are there rules concerning the interaction between a creature and elevated obstacles? Example: Say, you have the following. Tall Goliath (7'8") Short Gnome (3'4") Short Kenku (5'0") Short Human ...