I have been obsessing over the other fic that I lost, and I have remembered a bit more. Snape and Harry are stuck together in Grimmauld Place. Harry comes down one day to spaces potions lab in teh house. while talking to snape, he picks up a beaker or something (that he thinks is unicorn blood) and plays with it. Snape looks up and sees what Harry is doing, and he shouts No! making harry jumps, which spills the mercury on his hands. Snape rushes over and washes off the mercury, knowing it's no use, but, lo and behold, Harry doesn't die like expected. Snape, decides that some testing is in order. He has Harry drink various poisons. Harry eventually starts feeling sick, but only because of how full he gets from all the liquids. I think that they eventually discover that if he wasn't awesomely hard to kill he would have died in his cupboard long ago, because of fumes or something. I also remember a part where Snape and Harry are practicing with swords, when Harry gets distracted, so Snape ends up accidentally stabbing him in the heart. Snape is horrified, but when he pulls out the sword, Harry ends up living.
I think, that in this one, Snape has harry do some sort of work out, using a treadmill sort of thing, but magically. Early in the training Snape discovers that Harry has asthma, or something like it. He fixes that with a potion.
Also, maybe, the Order catches Bellatrix, and there is this whole scene, where Harry is pissed, and wants to kill her. He maybe does too.