The Lord with 4 thrall wizards should not be a problem.In fact,it should be a liability for him as his Lord will no longer be an IC and hence will be vulnerable to everything you can throw at him.Thrall wizards are merely T3 W1 with a 6+ save so anything in our arsenal can deal with them,the only thing I'm unsure about is whether he can use them to make chaos spawn even if they're dead(I doubt it though).Now,it would be best if you could cut them down till he has just 1 thrall wizard left along with his Lord,the bare minimum for his lord to not be an IC because what you could do then is a)markerlight them and b)use the markerlight hits to ensure sucessful delivery of a pair of railgun shots,preferably using XV88s as twin-linked BS5 shots are very difficult to miss with barring the most appalling luck.If done correctly,the Lord will have to make a 5+ Inv save(if applicable),or just die outright(unless he also has D.Rune which many do not take due to its prohibitive cost).
Rhinos should not be a problem to deal with,and once the rhinos are down the marines' mobility should be curtailed,enabling you to gun them down at your leisure.The Defiler if it hides behind some area terrain and uses indirect fire all game,can be assasinated with the usual suicide deepstriking Sunforge XV8.Otherwise,simply deal with it as we deal with most armor.
EDIT:After reading Wyddr's reply I realised that due to the majority toughness rules it might be possible to instakill his Lord with plasma rifles.You read that right.Not 100% sure here as I'm not sure of the exact wording,but if legal you could use it to 'punish' him for making his Lord vulnerable.While plasma rifles may not be exactly what you use,it is no doubt useful to know that he can be instakilled by S6 and above weapons,as you have probably noticed Tau can have a plethora of S6 and S7 weapons.
In short I advocate killing him off with a healthy dose of heavy firepower and some cunning.The same kind of cunning as when you position a crisis team so that you can only see and kill the heavy weapons guy in,say,a tactical squad.