Being able to do 50+ damage a punch is helpful, as is Combat Advantage! Once you have the deer trapped and fleeing, you can take your time. All is well and safe, and if you can't do enough damage to beat it down, you can just let it go and continue on your way!
In the case that something does go terrible wrong, it's advisable to have a boat as an escape route ready.
1. Find your deer.
2. Find it near, or within running distance, to a inner-corner river (image below).
3. Activate Oaken Stance.
4. Place down construction posts on the corner of the river in a small box shape - make it several deep.
5. Engage combat and run to the corner of the river - make sure the deer blasts through a few of the posts, then maneuver it so it goes down a right-angle junction of the posts.
6. Sling it asap so it runs.
7. Disengage.
8. Rebuild posts around it.
9. Re-enage combat.
10a. If it fights back, beat it down once more so it retreats. Do not disengage. Continue with guide.
10b. If it doesn't fight back, continue with guide.
11. You should now be able to melee it. Work up your Iniative (dash), and then Flex + "Knock His Teeth Out".
This guide works for me, not sure if it will for others - hope it helps anyway!
Corner post Layout:
~~~~~~~
A~~~~~B
XXXXX~~
XXXXX~~
-----XX~C
-----XX~~
A. This is where you should run to.
B. This is where you should wait till the deer kicks through the posts.
C. This is where you should try to lead the Deer to. Then when you disengage, box it in with a cornerpost at it's only escape.
This was my latest, lucky capture. Not sure how the post layout worked, but it did, heh. Looks like you can (risk) using yourself as the escape-blocking post, though.

My Iniative build-up whilst writing this guide. oops;



Addendum;
You can also try to gate-trap them. Heh. Same method; scare them into retreat, then have fun.
