I usually run 2 of them when I can with Full
CCW.
To use them, walk them up 6" a turn (unless they would be getting in range of something that could kill them) and shoot the battle cannon until you are close enough to fleet and assault.
However 3 main points:
1) They are scary to your opponent. He will shoot at it. A lot. Either give him scarier things to shoot at, or make the Defiler the pincusion while the rest of your army slips by unharmed. At 150 points they dont leave a mark on you when they die.
2) When getting into combat, try to stay away from power fists and equivelents. Juisy 10 man Marine squads with a power weapon is what Defilers love best. However running into a simple 2 attack Marine Sgt isn't the end of the world, you will probably survive. But you MAY not so keep an eye out for alternative targets.
3) Dont shoot at stupid things. I have seen rookie players so scared of an enemy's terminators that he would have his 2 defilers sit there and shoot at them all game. anything with a 2+ Save, YOU DONT SHOOT
AT! PERIOD!
Hope this helped