By Kroot force do you mean a Kroot heavy Tau army, or a Kroot merc list?
I only have experience with the latter (And that was a long time ago).
*In fact, I don't even know if the Kroot army is legal any more.
But I'll share what I remember from my Kroot Merc days:
Hug cover. Kroot are good at moving through forests, with their low save and average movement you need to limit casualties wherever possible. Abuse cover saves and unit visibility from forests where possible. Also, there's nothing quite as satisfying as charging from a forest into your enemy.
*With 5th Ed. and the run rule, Kroot will have an easier time closing the gap.
Most people seemed to assume Kroot were almost as bad in a fight as fire warriors on their first encounter (shows how often these guys get fielded in my gaming group...), so getting a pile of attacks on the charge at strength 4 from a swarm of Kroot often made cocky smiles fade.
Admittedly the Kroot aren't amazing fighters, but they are hardly awful. Especially when supported by a couple of Kroot hounds. (Don't waste your time on Krootox too much, they are expensive, and somewhat awful unless you want to pop light tanks with their gun, which the Tau have so many other weapons better suited to doing it's just not funny)
In the Tau list, the Kroot will benefit greatly from XV8 crisis suits being able to tackle units that the Kroot would otherwise balk at (Striking scorpions, Ork Nobz, Wyches), as well as the Tau's general excellence in reducing tanks to scrap metal.
For the kroot list itself:
Every unit has to have a shaper (naturally), give the shaper an eviscerator. He can tear up tanks (2d6 armour penetration, love it). Just bear in mind that aside from Krootox squads, shaper councils with meltaguns, and maybe some of the FW bits - this is about your only anti tank. Enemies will fear this, as pretty much EVERY squad in your army will have one.
Just beware that massed guns and elite melee fighters will give you a headache. You're basically slightly more expensive Orks with no troop transports, less anti-tank and more style.
Stuff like the Leman Russ will fill you with dread, as your only options fighting one are to run your shaper squad up the board and try to melta it, try to get a lucky Krootox shot on its side, or charge it with a Kroot squad. Not really enthralling prospects, as it can blow away chunks of your army (or your entire shaper council) in one shot.
I had around a 50% win rate with my Kroot, every unit can fight to an ok degree, their shooting isn't awful. They just lack heavy weapon options, transports and the amazing combat elite other armies are graced with. Every model in the army is just a solid fighter, no more, no less.
And remember, some units can be upgraded to use fleet. Take this where possible, it really helps.
Also, I heard sniper rifles get rending or something in 5th Ed. If the army IS still legal, the squads of Kroot snipers (yes, they actually have sniper rifles) could be barrels of fun.
If someone could let me know if the Kroot merc list is still legal, I may dig out my Kroot. I enjoyed that army...
