As per the title. The specific example I'm thinking of is an Archer Battle Master Fighter using Trip Attack on a Druid shapeshifted into an Air Elemental floating 120 feet off the ground. If the answer is different for this and the more general case, then answer each separately.
As noted in the tags, I'm looking at strictly RAW here, not RAI or house-rules, so references to specific pages of the PHB are appreciated. If there are any official answers on this, links to those would be helpful as well.