Specifically for dealing with strong early invaders, I would recommend:
- Warding behind the buff to spot them coming early
- Start at your buff camp rather than doing Krugs/Gromp first so you can't be caught at it.
If need be, clear one side of the jungle and back to heal up. While he might steal your other buff, it is still better than getting caught at low health doing it and dying to feed him double buffs.
It can also help to avoid getting a leash from your allies in certain cases or to start on the opposite side of the jungle. While it might sound a little contradictory (since you'd be at lower health and have a weaker clear), experienced jungle players can often tell where you started by when your top and bottom laners enter the lane (and whether or not they are missing mana). Denying them that information can often be worthwhile.
General Tips:
I would suggest watching videos by high-ELO and/or professional junglers (i.e. Nightblue3, Stonewall008, Valkyrin, and foxdropLOL). They will typically provide a lot of useful tips while playing and you can often see or hear their decision process and some optimizations throughout the game.
Off the top of my head, some basic optimizations may be:
- Use hard-CC when the monster camps are about to attack you to cancel that attack and delay their next.
- Each of the smite buffs are quite useful in different situations. Be sure to take proper advantage of them.
- Raptor's buff is great for ganking
- Gromp buff provides extra damage during clears and fights, especially for champions with a lot of health
- Krug's buff reduces the damage you take while clearing and lets champions without CC kill Scuttle Crab faster
- Wolf buff is useful when you are being counter-jungled or invaded (or after your initial towers have fallen).
- If you have a weak early clear, Krugs -> Raptor's -> Red Buff is probably the route that conserves the most health.
- Kill the little Raptors and the little Krug first on single-target clearers. They deal a significant amount of damage for their health value, so you will be healthier if they die early.
- If you think the enemy jungler will invade you early game, it can be safer to start on a buff camp rather than a little camp since you are less likely to be where they are looking.
- Attack Speed is extremely efficient at clearing on most junglers. I'd recommend using about 15% on champions when you need to speed up their clear even if they don't use the stat much later (with a few exceptions, such as Nidalee and Evelynn).
- Keep a pink ward out (or in your inventory) whenever possible. This is important for every champion, but I thought I should emphasis it. There isn't really a good reason not to.
As far as ganking is concerned:
- When possible, conserve your gap-closer while ganking. If you can get to the enemy champion without using your mobility spell(s), then you can save it to catch them after they try to escape (Flash, dash, blink, etc).
- You should should try to stay between the enemy and their escape route. You should be trying to move backwards (towards their escape) in-between attacks so that you can body-block them and slow their escape. This also leaves you closer to them if they try to dash on blink past you.
- Use Raptor's smite buff to determine if you have been spotted by wards when ganking to reduce the risk of them escaping or receiving a counter-gank.
- It can be risky to gank top-lane while Dragon is up since the enemy team can just group and take it 4v3. I'd recommend ganking top-lane early if possible (after your initial clear) and when Dragon is down; that lane can be quite snowbally due to isolation, so you want to get your ally ahead.
- Learn to recognize the general times when buff camps spawn during games. If you see the enemy gank bottom lane while their top-side buff is up, you can go steal it from them.
- Watch when top and bottom lane initially go into lane. If that lane has leashed for their jungler (missing health, mana, or showed up to lane a bit late), then you know where the enemy jungler has likely started and where they will end their clear (where is closest for them to gank).
- If you have killed the enemy laner, you can either allow your laner to freeze or shove the lane into the enemy tower. If you shove the wave under tower and the next minion wave is coming up, it can be helpful to proxy it to ensure that the wave fully resets and the enemy laner misses as much as possible. This is very situational though and you will have to learn when through experience.
Other tips:
- Whenever possible, walk up to the enemy champion during a gank and save your gap-closer for after they use their escape abilities.
- Watch the enemy laners when they go into lane. Whichever lane shows up later and/or missing mana will likely indicate which side of the jungle the enemy jungler started on (which is useful for predicting gank paths and for counter-jungling).
- When against aggressive early counter-junglers (i.e. Shaco) it can be a good idea to start on your buff camp and/or to start on a different side than you usually do. If he invades your buff, you've already taken it and/or you aren't there to be killed.
- Watch the minimap to see when and where the enemy goes to ward, which laners are pushing, which laners are vulnerable
- Know, roughly, how most laning matchups go so that you can prioritize which lanes are the easiest to gank or require an early gank to decide the matchup.
- If you have blown an enemy's Flash during a gank but haven't killed them, swing back around immediately after an gank them again. Many people don't expect it
- If you've killed an enemy during a previous gank in top lane, kill them again immediately after they return to lane. They likely won't have wards up yet and a second back-to-back death can be enough to completely crush any chance that opponent has to recover the lane.
- Top laners rarely bother warding the lane bushes. As such, lane ganks can be quite surprising and effective.
- When you kill an enemy laner, shove the minion wave into the tower so they miss gold and experience and so that the minion wave will reset to the middle of the lane. If you can get away with it, clear the upcoming minion wave as well to ensure the reset and maximum missed minions.
You will learn a lot through experience, but these tips should help you at the start.
