Okay, the top tier abilities thus far are

Entomb: Command the earth to rise up to swallow your enemies.
Prerequisite: Geomancy
Elemental Requirement: 30 Earth. 

Hurricane: Change weather patterns to create a full blown hurricane. The eye is formed at a targeted area, but the Monk has no control over the storm after creation.
Prerequisite: Thunder Storm
Elemental Requirement: 30 Air. 

Fusion:Starts a non-sustainable fusion reaction, devastating large patches of the battlefield.
Prerequisite: Vaporize
Elemental Requirement: 25 Sun

Still need one for Void, Water and Fire. There are also balanced element abilities that require both in full equal measures, but I decided to make them not as powerful because characters using both elements have more versatility.