How to deal with Critter Concealment Power?, One of my players is playing a pixie (sapient critter) Options

[Grimflayer]

One of my players wants to play a pixie, a sapient critter. It's a tiny elflike creature
with some critter powers, most notably the ability to fly and the "Concealment" Power.

Critter powers can be switched on and off at will and have no Sustainment penalty.
Also Concealment is not possible to dispel it and it works against astral perception too.

Since the char has Magic 6, concealment will work as a permanent invisibility spell (-6 penalty),
without the few flaws that spell has.
So i have a flying spellcaster that can hover invisible over a battlefield and all opposition
can only Blind Fire at -6 at him.

I'm somewhat concerned, we are starting a new game, and at the moment not all of my enemies
even have 6 skill to attack with.
I can raise the level of opposition, so they have more skill to have at least a chance to hit, but i
fear i will kill my other players doing so.

I would be grateful for a few tipps on what to do. Perhaps someone has had a similar situation?