Well, this is my first guide...
I tried the above solution with the boomshields in the stairways and It did'nt work out for me. What I did find though is this:
On waves 10, 20, 30..... so on (the levels with bloodmounts) If you're having trouble with the mounts the trick is to kill the bloodmount but leave the Grenedier Elite ALIVE, he will generally run away and take cover. After doing this with 3 - 4 bloodmounts your left with a gaggle of G Elites. While they are on the map no more bloodmounts will appear and you can concentrate on mashing grinders or maulers (wichever you find easiest to deal with). I leave the grinders alone and stay clear of them and batter the maulers instead. Occasionally I'll kill an Elite to bring back a bloodmount.
Be careful near levels end though (red bar changes to the number 5) I've come unstuck many a time with the seemingly random selection of baddies thrown in for good measure, a flamer here and a ticker and a wretch there!!!