In my last game, I think I had a glimpse of the (admittedly bleak) future for my Mournfangs as the High Elves poured four rounds of ranged fire into them from 3 archer blocks and 3 bolt throwers. I lost 3 of the 4, leaving only my Crusher.
I won in a Massacre.
While it's a hard life, the Inter Nets have made all and sundry aware of just how horrible it is to get charged by Mournfang Riders, so we can expect them to take most of the shooting/magic/redirecting nonsense until they're off the board. What I tried to do in that game is protect them from things that would have A) moved them out of position, or B) removed them from the table entirely. They're tough enough to force a shooting army to use most, if not all of its shooting for several rounds to get close. Rounds in which the rest of your army will get to explain why it hates to be ignored.
My recommendation is to keep sending them up the gut with 4 and a banner. The effort to kill them will, hopefully, consume so many opposition resources that many of your other units will cross the table virtually unmolested. If your opponents catch wise and try to adjust their strategy, then they attack the Mournfangs less...with predictable results on that, as well.
Keeping them near your general/BSB will make them last even in the face of concentrated firepower.