They are in a warp rift/storm right? That Deamonship is going to be next to impossible to kill if so. I think. Generally deamons are much stronger in the warp, to the point that the RT might need to get the Deamonship into range of a Geller field which is beyond close range and practically touching at that point.
Basically unless their fleet is heavily damaged then I suspect the RT will be forced to retreat.
Here's how I see the fight going down. The RT surprises the Chaos fleet with it's tactical FTL doing some damage. He gets in some more hits but then the Chaos Fleet positions itself so there is no safe place to attack from. The two Merc frigates go down to a barrage of fire. They trade blows for a little while and the Lunar decides to bugger off. The RT is forced to retreat shortly afterwards. Chaos loots the destroyed frigates.