They're most powerful psykers when it comes to 'normal' species. A Swarmlord (which I'm guessing this is meant to be) is very powerful too though. Also the disembodied spirits of eldar have nowhere near as much power as when they're flesh and blood. Also there's the shadow in the warp which will be messing with their heads big-time.
In terms of power a farseer is as strong if not stronger than a hive tyrant, but this seems to be an extremely powerful hive tyrant that the hive mind has created specially to fight eldar, not that it needs a lot of help with the shadow in the warp.