I am an experienced Descent 2nd edition player and very skilled in playing. I win way more often than I lose. I have tried all of the Co Op adventures and find them very difficult to win. In fact, I have lost them almost everytime played. Any suggestions on what I may be doing wrong. To give the heros a fair chance to win, I have implemented a house which says a peril card is not drawn if there was an active adventure card at any time during that round. Without this house rule, for most games, I go through the peril deck 2 full times or often more. The heros I typically use are the female warrior with knight class, the necromancer, and alvric with the healing class.

They are difficult, but doable. I dont like necromancer much. maybe a runemaster can be a better option I dunno...

Try playing with 2 heroes. In OL games, those are the toughest. With the app/coop modules, that makes it easier (especially with the 2-hero attack rule).

In general, the cooperative game modes are intentionally more difficult. Since the opposition isn't as smart, the game is made harder to account for this.

I have helped to design the coop quests, and played them like around 40-50 times the first one 20-30 times the second one and around 15times the last one.
From my observation the first one had a winratio about 95%, it is pretty easy and manageable if you know how to game the ai and the roommechanics and the traps.
The second one is much harder than the first one and had a winratio around 60%. Effectively managing your characters here and not wasting any time on any room is pretty much required.
The thrid one is a really hard beast to tacle and we only had a winratio around 25%. Sometimes random things or specific sequences of rooms can lead to a quick death. But this was also intended to be the hardest one.

A couple of hints how to solve the coop adventures:
heroes with high movement high stamina are premium.
get hero classes that have alot of free passive boni, mobility or free actions such as door opening. Especially the thief is pretty good in the coop adventures as he can grab chests from a distance and open doors for free.
Also look for heroes that have balanced attributes, you will need to roll them! Lindel is extremely premium here for many objectives.