Gonna throw out some plans that aim to destroy Turoq quickly.
Plan Just Attack:
1. Leave just enough forces at home to fend off Turoq attacks.
2. Attack Turoq's planets with the rest of our forces.
Turoq's ships may have great speed through the Warp but their infrastructure and population centres can't be moved. Their navies and armies would still pose problems afterwards but less so than the civilisation would if it was left alive. We'll take heavy casualties, but heavy casualties is typically a small price to pay for denying Tzeentchians time.
Plan Eldar Help:
1. Bargain with the eldar to give us access to the Webway so we can attack Turoq.
2. Attack Turoq.
The problem here is the whole 'eldar diplomacy' thing. That could fuck us over but the alternative is leaving an entire Tzeentchian civilisation to do whatever it wants for who knows how many decades. Much more chance to get fucked over and fucked over hard.
Plan Surgical Strike:
1. Send a relatively small, super elite army to assassinate Turoq.
2. Fan the flames.
This plan requires psykers, Astartes, Titans, Last Hunters, and, crucially, a soul-killer (or several). We use Ridcully's talents to plan out a perfect assassination on the lynchpin of the civilisation and end it. Ridcully then fans the flames on the inevitable civil war so we don't have to deal with too much of the remnants' shit. One advantage of this is that it lets us use Avernite assets such as sirens, formics, and Areatha since quality is a lot more important than quantity. The biggest disadvantage of this is that it risks a whole lot of our best assets on a dangerous endeavour, and Ridcully can only do so much to make it safer.
Plan Eldar Mercs:
1. Get the eldar to kill them.
Requires giving a big favour to the eldar and they may not even accept.