and the fact that you have 0 supplies, no medkits, no bullets, except for 9 or 7 arrows, you rely a lot on your bullet drops from enemies. Sometimes enemies will drop 1 bullet, sometimes maybe 2, but sometimes none... and you have to be lucky with that, because:
a) it takes a lot of time to reload
b) if you ran out of bullets when there is a big wave you are simply a goner
c) David is fucking USELESS
So luck plays quite a big part.
Oh and you shouldnt miss. 1 miss fire is ok. 2 hmm ok you can get it done if you are lucky... 3? Better reload checkpoint
A lot will depend on your bow accuracy as well.