I had the INFINITE pleasure of facing the Guard gunline list (i.e. Leafblower and variants) a total of 6 times at Adepticon - over 1/2 of my games. My record against them was 3 wins and 3 losses. No less than 11'sh points in any of those losses as best I can remember - so maybe more of a draw than a loss. Max I got was upper 30's against them in one game - upper 20's in the other two.
Here's a few things that worked well for me - both with my Eldar and with Marines.
a) ALWAYS let them go first. Put ALL your stuff in reserve. Few if any armies can withstand getting swatted one turn - minimize the number of rounds you will take fire. Deny them targets and turns. I was never tabled and went 7 turns on at least two of my games. A plus to this is it takes them out of their game - what to do when they have no targets. Obviously this is a tactic best used by Eldar with an Autarch - but worked reasonably well when facing them with Marines.
b) Speed and maneuverability - if you can outflank or have things that can come on behind them - use them.
c) they have relatively fragile troops - focus on them. Pop those Chimera's and then rain hell on the soft stuff inside. Only worry about the Vendetta's when they have passengers - I found them most difficult to drop personally. Eliminate their ability to control things.
d) It's a hard list to get max points off. Pick one or two of the victory conditions and focus on them. The opponent will most of the time focus on the primary - so go for secondary/tertiary - the points still stay good and keep you in the running at least.
It's a tough list - a real tar pit to play against. I would say that roughly 1/3 of the armies I saw at Adepticon for all the tourneys had Guard. That's not scientific - but they were everywhere.