If farmers ever parried I would be screwed. My weapon of choice is remarkably easy to parry (great scythe). In all of my pvp with it, I found one guy who ever parried. He did that to my first attack, and one shotted me. It was embarassing.
By the way, I've learned that while dead angling is great in duels and against single opponents, it's a bit meh against farmers. Personally, I go for quick long range swings to chip away at their estus for another invader to kill them. Also, if your weapon has a really long swing (in that it goes behind you), don't worry too much about getting bs'ed. A good player will crush you, but the farmers I fight tend to circle fairly close to you, and they'll get hit and hopefully staggered by that. It also seems to knock them off their game for the rest of the fight, or at least a few seconds. They'll be more cautious, and won't take the bs opportunities that you provide by trying to kill the other enemy.
Also, at least in my experience, being overly cautious is worse than being overly aggressive. I'm way too aggressive, but at least that way I might surprise them and get a bit of damage in, causing them to waste something. Also, don't feel too bad about losing as long as you got some damage in. If they heal, you made them waste some estus, if they don't it's that much less damage that another invader has to deal to kill them.