Angel of purity isn't that much harder than the other heal restricted challenges, all you need to do is play a decent game of DoomRL (If you've beaten HMP then this shouldn't be too hard)
There is one thing to remember. While it's usually smart to not heal unless you desperately need it (and not carry many medpacks anyway), in AoPu you might as well use your small medpacks the second you hit 70% HP because there's no way to heal aside from medpacks. Still save your large ones for much later though, of course.