Learn English – To pinch or to grab someone’s face

The adjective form of various shapes ending in "angle" is usually shapename-ular.

So, for example.

circle -> circular
rectangle -> rectangular
triangle -> triangular

Square is an exception. The adjective form of square is just square.

The correct form of what you're trying to say is

Her face is triangular.

Though I don't think someone would take kindly to that comment, so I would keep that comment in your exercise books only.

Neither option is really incorrect from a grammar standpoint but a native speaker would not use either of them. We don't say that someone "hits on someone's face." On is used more when your face (or other part of your body) hits some inanimate object, like "I smacked my face on the open cabinet door" or in your example "I hit my knee on the table."

Instead we use "with" to indicate the object used to hit your face:

She might hit your face with her elbows.