As Tiamat, defeat two enemies in each of her stances in a single match.
Tiamat is a stance switching goddess, switching between a ranged flying stance and a melee grounded stance. You switch between stances by using Tiamat's third ability, Grounding Dive/Rising Flight. For this achievement, you simply need to get two kills in each stance in a single match, so a total of at least 4 kills in a match. It doesn't have to be on the same god. If you're playing at launch, she'll likely be very OP and this shouldn't be at all hard. Later on, after balances, this may change.
You'll be most likely to get this in Arena or Assault, as they are both just more aggressive modes and will give plenty of kill opportunities. You'll want to build high power magical items, as well as some appropriate protections for your melee stance.