I have just got a Luxknight. Looking at the skillset, it is an all-out offence but lacking a strong single or AOE attack (i.e. no 250 TU skill).

Is it worthwhile to train it? It is a 9.5 stars, but its skillset does not seem to be at a 9.5 star level?

Yes it is.
It has a very good triple target ability that can take down almost all the health of a good shadow type ark.
I’m currently leveling it to 99 and it is a beast in pvp.

It also has a decent defense stat, which makes it really nice for PVP especially

very good ark, takes out most shadow arks with one hit.

Basically, you really want it for PVP until you can potentially get a Guardiron

Im also lvling it up to 99 only then can I see if it has any value to me, u should do the same for urself