Vs. Flanking Mech IG

How do you guys handle flanking mech ig? This caused me a little trouble in my last game and using a DE parking lot didn't seem to be the best strategy. Over half of the army flanked or deep strike. Should I have just reserved everything?

Honestly depends on what was actually flanking. Any chance you could post a rough outline of his list? Mainly looking for numbers of things rather than detailed lists of upgrades and the like.

Well to be honest I don't know the names of the vehicles. He flanked two of the large planes (storm ravens I think) and two troop transports that resembled rhinos roughly. Each carried ten troops. Then he deep struck three parachuters which I was able to manage. If I recall correctly he also flanked with one of the smaller tanks. Sorry I couldn't clarify more. New player and never read through the ig codex lol

Flanking IG - versus DE, is generally less intimidating than other types of IG (in my opinion at least).

Was it the alpha strike that he got that hurt you so bad?
During your turns did you set up to move flat out for cover and be away from board edges so he would have a harder time hurting you?

I did not. I turtled back in the corner behind a small bit of cover and made him come to me so I could focus down what ever he brought in close. The biggest issue was that his first round dropped both of my ravagers leaving me with my razorwing, Trueborn, and raiders as the only form of at left. Even moving flat out with my ravagers I would have been pretty much in range depending on how he rolled for the flank. Maybe I should just reserved them next time?

By turtling in a corner you're putting yourself in a situation where 66% of the time he'll have you in range guaranteed. By flat outing towards the middle you almost assure yourself to be out of melta range - which is a nice melta vet counter. Also, turtling isn't even the best way to defend from DSing either, optimally you want to spread out a bit more, fill in open areas, and make him risk mishaps when DSing in. A dirty option is to, of course, just line your troops up all along one side of the map cordoning it off - but that tends to not be a good idea if this is remotely friendly play.

Reserve is most assuredly a fine tactic versus an outflanking army. I personally don't do it much because I prefer to go with the extra firepower in reply, but if you're not confident in your placement/are playing on a wide open map, going reserve becomes a very sound strategy to help lessen the impact of the flank move. Also, since we're so fast, it doesn't hinder us as much as it does other armies, making it a stronger option for DE as compared to many other codices.