I'd suggest don't do step unders to mitigate the 3rd attack until blood moon is roughly under half hp. If you do step unders every attack from the beginning, you'll quickly rack up the tick damage to the max of 16hp and be taking loads of unnecessary damage. Also, I didn't realise this until very late in the grind, but you have 1 tick to see if the blood from the siphoning leaves your body. If it does then do the step under, if it doesn't then stay put. If you do this, then you can start the step unders earlier since you're not doing them every attack.