First off, Galak wrote a pretty good playbook. See the link below:
viewtopic.php?t=7779
I play Halflings on FUMBBL, and I'm doing OK with them. I end-up scoring using TTM almost once per game. I also played against Halflings on table-top.
How I have been doing it is very similar to the playbook above. Always buy ingredients to steal rerolls from your opponent. I have 3 rerolls, so plus the others that I get (usually around 3), I get almost a reroll per turn, and ideally my opponent gets none.
Always keep halflings close to your trees. The trees provide protection for the flings. Move forward slowly, blocking with the trees. Hope that they don't take root too quickly.
Once the opportunity is there, move your fling with the ball close to the tree, hand-off to a fling standing next to the tree, and throw him. Having a fling with Catch to do this is quite useful. Keep the reroll for the landing. Note that with the tree, all you want to avoid is the fumble. An inaccurate pass is still OK. Try not to throw anything longer than a short pass, which is usually enough to get into scoring range.
If your opponent insists on trying to take the trees down, then go for the running play with the Halflings.
Keep your TR low by spending money on ingredients or hiring Deeproot if you have the cash. So unless you play against Dwarves, you will encounter very little teams with tackle, so the little halflings become very difficult to take down.
Skills for the flings: side step, diving tackle and catch (to catch the handoff before being thrown) are the skills that I try to give to them on normal rolls.
Try to get passing skills on your trees.
A play that I sometimes try is to throw a Goblin to break a cage. It is a desperation move, and is no that likely to succeed, but it sometimes works. If your opponent has caged-up and your Flings and Trees can't get in the cage, grab the nearest fling and throw him in the cage. If nothing else, it is quite amusing. And imagine the look on your opponent's face when the scattering fling lands on the ball carrier
And remember, always try to keep a reroll for the landing. The landing really is the critical part. Unless your little guy gets a really bad scatter, if you land, you score in the same turn.
With Orcs and Goblins, it is harder. The Really Stupid and Always Hungry Trolls are much more unreliable, and chances are you will have less rerolls available for the landing, hence making the TTM much less likely to succeed. It's not so bad with Ogres, but Ogres tend to run out of rerolls pretty quickly.
Still haven't figured out how to model the above scenario though 