A little advice for archers staring down the barrel of a Troll's rock-gun:
A lot of archers, in my experience, will be so terrified they either don't do anything (because they think they cant charge their bow in time), or run away.
No matter how weak your shot is, always shoot the Troll with the rock! It throws the aim wildly off balance in the split second before throwing the rock, and vastly increases your chances of survival
(Unless, of course, they are a Rocknoob-Troll, in which case you might inadvertently better their aim.)