counter-jungling safely requires good map awareness (although regardless of that knowledge, nunu can escape quite easily with W+E)...
Wards are your best friend when counterjungling, so you can see the enemy jungler approaching before s/he sees you. 1 or 2 points in consume should be sufficient. Max snowball first so you can chase/escape better and also it will be much more helpful if you decide to do some early ganks.
try to drop an early trinket to find out where the enemy jungler starts. you can also guess this information based on which champion they are using. For example, amumu or fiddle will most likely start blue since the mana helps them clear. An auto-attacker might be more likely to start krugs or red. With nunu, it's fun to try and get or guess this information, take your leash camp for level 2, and immediately steal their closest buff. Trinket where the jungle would probably come from during their clear. You now have timer and can take this buff every time it comes up if convenient.
other than that first interaction, pay attention to the minimap. if their jungle shows up on the map, you can now immediately invade safely with that knowledge of his whereabouts. still pay attention to the enemy laners in case you crossed a ward and they are collapsing on your position. Smiting enemy wolves is great and can keep you safe or simply give you the enemy jungler's position when s/he enters blue side of jungle.
also, nunu isn't the strongest ganker. not to say you can't gank, but they aren't super effective as all you have is ice ball and autos. If you can gank while having the red buff, your ganks will be much easier. But overall, nunu is king of counterjungling; if you can simply starve the enemy jungler (often leave 1 small monster alive so that when they do return to the camp, they will have wasted time walking there for the measly 20g), your job will be done. The jungler will be poor in gold and xp. This is especially satisfying against devourers and can basically make them useless if you manage to delay their sated past 25-30 mins. ESPECIALLY against devourers, make sure you take crabs and dragons. That will delay their stacking immensely. if you're up 20-30+ cs on the enemy jungler mid game you're doing great.
Then late game just peel for your carries with E and R. Buff adc with W. R is better used to peel disengage than engage. good luck, nunu is such a pest it's so much fun. Oh ya, you can usually just use Q as smite and save actual smite for the counterjungling. I wouldn't invade without 1 charge of smite at least, otherwise your clear will be much slower and increase the likelihood you are discovered. Lastly, when you do baron or drag, check Q and smite damage. add that up, when the monster at that HP, smite + Q at the same time and there's almost no chance for it to be stolen.
i don't know why i typed so much lol hope it helps
and
for jungling ( my first two ) and have found I like their themes but am struggling with their playstyles. Of the two champions I seem to be doing best with Nunu; farming my gromp or krugs and red or blue depending on which of the two are closest to botlane before marching taking the buff I neglected before and seeing if I can make my first early gank. What I'm lacking with Nunu is knowledge of counter-jungling to help put myself above the other jungler but by comparison I'm much worse off with Yi. With Yi my clears are weak, ganking pre-six is something I've learned to avoid and I really have no clue how to get involved in teamfights/how to stay alive during them. I'm here seeking advice on the best way to learn what I'm lacking with these two champions and am hoping for your help; feel free to ask me questions about my gameplay with these champs if you think it will help you give better advice.
