I have a lvl 2 aarakocra rogue with a shortbow player in my campaign, but they use the flying ability to disrupt enemy lines and become a challenging combat in to a trivial one. How I would made a combat considering that?

Have an enemy shoot the dude in his wing?  
Also a lasso can be a really dangerous weapon against a flyer, I’d rule it as a ranged touch attack to hit. Then have the flyer make a dex athletics check to slip the lasso. Target number should be whatever the roll was to hit him. 
That’s how I’d rule it anyways. 

Anyways, the lariat as a weapon of war has a long and storied history being used by cavalry in the Middle East and Byzantine area.

One more bit, don’t let him “just fly away” when enemy archers attack. He has a movement speed, and flying or not he can’t outrun a spell or a natural 20. 

Quote from DeimosMK >>

I have a lvl 2 aarakocra rogue with a shortbow player in my campaign, but they use the flying ability to disrupt enemy lines and become a challenging combat in to a trivial one. How I would made a combat considering that?

 Ranged attackers. Particularly effects that knock prone or immobilize, either of which will cause him to fall out of the sky.

The ability to fly is stupidly powerful at low levels.

Run more combats that take place inside buildings, or caves.  A 10 foot ceiling really limits what a flyer can do.  If you must run a combat out of doors, remember that trees, branches, and foliage really limit line of sight from above.  If the Rogue cannot see their target, they are at Disadvantage to their ranged attacks.  Not every combat needs take place on the wide open prairie.

Sleep is a 1st level spell with no save and a range of 90ft(+20ft). As long as you roll above their HP with 5d8, they'll drop like a stone.

If your player flies outside of that range, they'll be firing at disadvantage and won't have sneak attack.

If they're flying at 50+ft, they'll probably be immediately knocked unconscious. If not, everyone who shoots at them before they land gets advantage.

Are they outside all the time? Indoor spaces don't usually have high ceilings.

Any effect that reduces a creature’s speed to 0 will cause a flying creature to fall on its next turn, and a creature falls up to 500 ft. per turn. 

Things that knock him probe will drop him from flying, too. Lots of spell effects can do the trick. A battle master with a trip attack. Lots of others. 

Quote from Geann >>

The ability to fly is stupidly powerful at low levels. 

We have an Aarakocra in the party for whom I am the DM, and I have not found it so. He is 5th level now, so we've certainly done the low levels. He can fly, and yes has used it to tactical advantage many times. I do not believe he has been more significant in combat than other PCs.

Run more combats that take place inside buildings, or caves.  A 10 foot ceiling really limits what a flyer can do. 

I have actually on purpose tried to put the party in as many settings as I can that do NOT limit the Aarakocra's flying ability. Last adventure they were in an underground necropolis and I purposely designed the cavern ceiling to be nearly 100' high in part so that the Aarakocra would be able to fly around. He used it to good effect in a couple of the battles. I have no problem with this.

There have certainly been some indoor battles. But even then unless they are in some small cave or something, they have often been in places that have 20 or 30 foot ceilings. Again, if possible I try to do this on purpose because I feel that as an Aarakocra he should get to use his flight tactically as much as possible.

I mean maybe more of my enemies than usual have had ranged attacks... I guess? Although especially in the low level game I mostly used creatures straight out of Monster Manual.

I agree with @BioWizard. I’m running a campaign that has an Aarakocra in it that started at 1st level and I ignored his flying ability when I was building encounters. There were times when his flight made encounters trivial and there were times when his flight wasn’t a factor in the encounter. 

The danger of falling if he was reduced to 0 HP limited how much he used flight and intelligent opponents with ranged weapons targeted him as long as he was flying since their allies without ranged attacks couldn’t get to him. Unintelligent opponents on the other hand ignored him and he used that to his advantage!

My main reason for trying to provide as many circumstances in which the Aarakocra could fly is that there is little point to making up a bird PC if the character can never fly, or is punished every time he does.  Just like I tried to make sure the forest gnome got to do some woodland encounters and meet some squirrels and foxes and such.

As a DM, I find flying PCs to be emmensly problematic.  So I am very strict on weight limits, armor restrictions and flying mechanics.    I would have any thick forest require an acrobatic check to fly more than 1/2 speed, minimum.  Failure means save (to land) or crash (damage) where you are.

I have found if you are willing to retroactively say "intelligent creatures would be aware of flying threats and do X" you can easily fix modules that were written without taking them into account.

This lets flyers easily defeat many traps and plans of stupid creatures, but not creatures whose average Int >= 12

The other thing to consider is whether progression is based on XP or Milestone.

With XP, the combat advantage could be exploited to grind levels too quickly.

With Milestone, trivializing intermediate encounters is less of an issue, since they won't gain levels until they overcome significant challenges that are probably more complicated than flying alone will resolve.

Quote from Memnosyne >>

With XP, the combat advantage could be exploited to grind levels too quickly.

 

 I disagree. If there is no threat or challenge, then I award less exp.

For instance, there is a dungeon where part of it is guarded by creatures that cannot leave water. The party stayed away and just used cantrips to slowly kill them. No exp. No threat, no challenge, no creative solution, no exp.

The operative word was "could". I was referring to default XP progression, according to the values in the monster statblocks.

Modifying that value according to Effective Challenge Rating is certainly a good way to handle it, and should probably be discussed with players ahead of time. If a DM allows a player to play an Aarakocra without first explaining how experience will be handled, it may feel like the party is being punished unreasonably if the DM regularly serves fish in a barrel, rather than appropriately challenging encounters.

So, to sum it up:

• 10-foot dungeon ceilings
• Ranged attackers
• Flying attackers
• Grappling or other immobilization
• Thrown nets could be absolutely devastating
• And once in a while, just let the player have fun abusing flight!

I really think if the DM is going to only let the Aarakocra fly around in battle "every once in a while," then the DM should just ban them outright. I find it highly unfair to find ways to prevent a character from using the primary feature of the race. It's like saying to use magical darkness in most dungeons but once in a while let the Dwarf have fun using Darkvision.

That's not exactly the same. It be more along the lines of saying Elves can't use swords or bows.

If a single ability or player is destroying game balance, then it’s absolutely fair to limit those things. Talking it over with the player would be a good idea, too. Even if to just to let them know why that change was made. 

Quote from BioWizard >>

I really think if the DM is going to only let the Aarakocra fly around in battle "every once in a while," then the DM should just ban them outright.

 Honestly, Aarakocra are a poorly balanced race and shouldn't be allowed into campaigns unless the DM is very certain they want to cope with it (it would be useful to flag various races with 'STOP! Potentially campaign breaking, requires special DM approval'), and has the secondary problem that many of the methods available to monsters to deal with it will instakill low level characters.