Yeah, Heartland is weird. The map is locked to normal difficulty (which is a complete joke after you start playing on lethal), but then you get to the plague wall gauntlet and suddenly it's like LETHAL+++++. And yeah, taking a follower to a plague wall is pretty much RIP. I lost IzzBee that way on my first playthrough of Heartland and just started the whole thing over (I had also ignored Vicente's mission because it was in the ass end of the map, only to find out that shit times out super fast and once he leaves you can never recruit him or Isaac in that playthrough, so that was already grating on me.)
They're pretty easy really, just have to figure out what works and what doesn't. On anything below lethal just dump bullets into it; even on nightmare it only takes 40 .22 or 9mm which is easily done without having to reload if you have some of the better guns. On lethal, one road flare followed by 3 bloater grenades kills it and that takes like 5 seconds total. Road flares are an extremely limited resource though, so I normally just drop a C4 next to it, dump a mag of .357 (most efficient ammo type) from my MPX, blow up the C4 and reload, then it takes like half a mag more to finish it off. Total time maybe 10 seconds including the time to run up and plant the C4.
Of course the thing that actually trivializes the whole process is ScentBlock. With it active you can even just melee them down and spend almost no resources at all if you really want to.