Create a situation where he has to be both tangible and fully intangible
Like avoiding danger while staying on a platform.
(Please feel free to correct my grammar and prepositions without asking)
For example, attract the attacker on a ladder above lava (or anything else lethal), then trigger an explosion on him. He will have to turn fully intangible to avoid the explosion, but he can't do that while staying on the ladder. He'll fall through into the lava. Your rules doesn't say what happens if he falls into lava while being intangible but I don't really see how he could get out alive. And you might not even need lava. Consequently:
Making him fall inside the ground. 922
Falling from a high point might be lethal in all cases, depending on your rules. When he reaches the ground at terminal velocity(), if he's tangible he'll die instantly, and if he's intangible he'll fall inside* the earth (and will keep falling until he decides what to do!).
Then what happens, based on your rules, if he's inside matter and he tries to become tangible? Maybe he just can't, if so he'd keep falling towards the the center of the earth (eventually reaching the other side), but there's no way he can breathe so he'll suffocate in no time. Maybe if he does he dies instantly (or just lose a body part, if it's just a arm in a wall for example). Either way he's dead.
Ultimately it's your choice, but if you want to give the attacker a weak point it's a great choice, it makes sense.
Also, based on the fact that he can "walk" on any surface a normal human can, there's a choice you have to make. I see two possibilities: either he always keeps the sole of his feet tangible, or the ground magically repulses him. If it's the latter, he'd reasonably crush on the ground even when intangible, so I think the former makes more sense. But if he's falling really fast, making a small part of his body tangible won't save him.
(*) As a bonus, if he stays intangible while he falls he'll never reach terminal velocity because the air won't slow him down, so he'll keep gaining speed indefinitely :) 238