Hi,
I had similar problems before joining this site - but watching the videos really helped me improve my game and beat them.
My biggest problem was that these players were usually taking really weird hand, so I simply couldn't put them on any reasonable hand, and I also didn't adjust well enough to they way of playing.  For instance, I got confused quickly when the "standard" line of raising pre-flop and continuation betting on the flop didn't work because they either called all the time, or check/minraised me or donked into me etc.
One thing I really learned from watching the videos was taking non-standard lines against these guys to properly adjust to their weaknesses - for instance not raising any donk-bet with very weak holdings or how to adapt to loose 3bettors by minraising and/or limping some buttons.
In the past, it also made me tilt very quickly when a bad donkey won a match against me and I often did really expensive tilt-rematches against them where I did not properly adjust to their style and thus lost even more money.
Jack

Well, I watched about 20 of the videos so far, and I liked them all.
I started with PrimordialAA's Fundamentals series, then watched most of Skates' Videos and the lower-stakes ones from Croixdawg.

the biggest change you have to make against these players is just to cut back your agression by a ton, instead of raising pre and Cbetting the flop, knowing that he calls with a very wide range anyway, you can just limp and either take pots away by making small stabs (really player dependant) or just give up on your missed hands and valuebet hard with your good hands.
The reason why solid LAGs sometimes have it very difficult against these huge fish is because they take you out of your comfort zone, they take away your 2 biggest weapons in HU, raising pre and Cbetting, by just flatcalling leaving the raiser with no clue where he is in the hand. This can lead to a lot of frustration and multiple barrels with air.
So against the really passive players you really have to think about how big of a pot do you wanna play against them, take a lot of pot controlling lines like limping pre or not Cbetting no matter how dry the board is.
The same works against the overly agressive players, playing passive induces them to play even more agressive, wich means you can trap them very easily and make semi-light calldowns with mid pair hands and fold the hands that can't stand a double or triple barrel.
Hope this helps.

 Just gonna say Trippy is spot on.
And first thing I would do is start limping and/or tighten your open range.

That TrippyTrippy guy sounds like he knows a thing or two!
;-)

I've had a good teacher ;)

Good point.
Another huge problem I had against these extremely bad players was being overly aggressive with weak made hands and thus building too big pots with them.  Coming from a 6-max cash background, I've been used to getting raised / 3-bet with things like two pairs or sets - especially on very drawy boards.
I've also watched a lot of these higher-stakes heads-up cash videos on other training sites and I remember a lot of the coaches saying things like they bet 2nd pair for value on the river because anything better would have raised them on earlier streets.  Or where they eliminate hands from their opponent's ranges because they don't make any sense because of the action on earlier streets.
Unfortunately, most of these very bad players simply don't know how to properly play their hands, so they do weird things like check/calling you two streets with top set on a very drawy board and then go for a check/raise on the river, where all draws have missed - and I ended up calling off my stack with top pair good kicker because their line simply didn't make any sense.
I've also raised way too many donk-bets with too weak holdings before - most of the time, I was ahead of their range at the flop, but when they hit their miracle card at the turn, they got my entire stack.
Watching Croixdawg's videos and also Primo's Cake one really helped me improve my game against these types of opponents.
Jack