100% invade is only in Huntsman Copse near those big fat guys. His other invasions are totally random, he can invade almost everywhere, including DLC's. He can invade you even multiplie times in exact same area, example: He might invade you near every giant tree in Fotfg. He can invade you even after you defeat boss of the area or even after you clear whole area from the enemies. He's an ******, but there's one thing to make fights with him easier, he block with sword/scythe very often, like about after 2-3 attacks/combos/hit-trades, he will block for some time, enough to guardbreak and punish, take that atventage.
IMPORTANT: There is one nasty place where he can invade you - it's in Shulva. Inside the sanctum in room, where are multiplie ghosts. Near hidden bonefire (the one you have to open with those buttons and jump for it), in room where there are cursed corpes of knights you want to destroy to make them valueable on physical side. The room where are two big holes and one little path beneath those, the corpses are on walls. YEAH, he might invade you in there with 3-5 ghost at same time. Here's tip, just run around and destroy a corpse at a time when u can, dodge, knights will eventually drop in holes, when u destroy all of corpses, kill knights first or try to make them fall, then kill him.