I've only been playing Titania for a month, but I've noticed a few things in my games. Positioning is very important. You want your underbrush markers getting in your opponents movement lanes and interrupting sight lines to get negative flips on non melee attacks. You also want your opponents within two inches for the into thorns trigger that most of the keyword has. Being able to place an enemy model on the other side of the marker can free you up to move without disengaging.
Aeslin and waldgeists are really good if there's a large severe terrain piece on the table, since they can attack any model in it with them regardless of line of sight or distance.
The autumn knight's are very cranky and take some effort for your opponent to put down.
The biggest problem I've found is speed. Titania can outpace the crew easily, so be careful not to let her get stuck out on her own, especially if the enemy has ranged attacks that ignore concealment.