I don't understand how to get the "three for three" medal.  It says I'm suppose to take down 3 henchman with 3 different walls at once using explosive gel.
 
So I assume that means I have to put explosive gel on 3 different walls and then detonate them all at the same time which will take out 3 henchman at each wall.  Really?  That seems absurdly impossible because I cannot, for the life of me, get 3 different enemies to stand where I need them to stand. If I'm lucky I can get 2 guys near walls but the level that this medal is on isn't exactly ideal for this situation.  
 
Anyone got any tips?  These predator challenges are more annoying than the combat ones (I've already got all combat medals at this point).

I followed this dudes vid for it 
 
  

thx!  I'll try it out
 
edit: Got it done.  Thanks for the vid.

I did this without videos and my final time was about 1 hour :o