Placing a Demacian Standard increases the stats of allies as well as one's own. Remember that Jarvan IV gains both his passive's and the standard's bonus stats.
Placing a Demacian Standard in a team fight can be very rewarding, as not only do you aid your allies in battle, but you also get more assists whenever one of your allies kills an enemy while having the Demacian Standard buff (even if you didn't deal damage to the enemy champion).
Using Cataclysm to cage multiple enemies is a great way of dealing damage to them without harming your own teammates. Be careful where and when you place it, as it may not turn out to be in your favor. Using Cataclysm at the wrong time/place can cause unnecessary death to you or your teammates, for example caging the wrong enemy out or caging low-health allies within.
Enemy champions such as Shaco, Ezreal, Shen, or other champions with a similar short-ranged teleport/dash ability (including Flash) can easily escape Cataclysm. Be aware of this when using Cataclysm on these targets.
Be careful when using Cataclysm on enemy champions that do high AoE damage to enemies around them, such as Amumu or Kennen, or in range of enemy champions that have ranged AoE abilities, such as Brand or Malzahar. Accidentally trapping your teammates with or within range of these types of champions will make it so they take full damage from otherwise avoidable AoE abilities.
If built to survive, Jarvan IV can start a team fight with great success. He also deals damage well without many items. While building him, strive to maintain both survivability and damage. A team with skills such as Bandage Toss can be used to get inside the Cataclysm and complement Champion ability to initiate well.
Combining a well-placed Demacian Standard with Dragon Strike can disrupt the enemy enough to turn the tide in a battle. This can also be used to chase an enemy or cut them off by pulling yourself across terrain.
When Cataclysm is on cooldown, you can use the Demacian Standard and Dragon Strike combo to help your teammates escaping anyway. Wait for the enemies to walk past you, then use your combo and Golden Aegis immediately after to stop nearly any chaser in their tracks.
Building a Blade of the Ruined King will significantly increase Jarvan IV's DPS (the passive and attack damage alone, without the passive and attack speed, will be more effective than a full stacked Bloodthirster versus targets above 3000 maximum health) and improve his burst. The passive, combined with Martial Cadence, will deal an incredible 14.5% of the target's health on the first attack plus your Attack. Combined with the active, one can potentially deal almost 30% of the target's health + basic attack damage without using any abilities. The active also adds another form of CC to Jarvan IV's already impressive kit. Finally, with his high natural Attack Speed from Demacian Standard Jarvan IV will be able to benefit greatly from the passive effect and lifesteal.
Blade of the Ruined King grants very little Attack Damage though. If it's Jarvan IV only damage item, his burst will suffer slightly. An Atma's Impaler on full Tank Jarvan IV will grant about 70 AD, which adds 70 + 70 x 1.5 = 175 damage to Jarvan IV's E+Q+R combo, while a Blade of the Ruined King adds only 25 + 25 x 1.5 = 62.5 damage. Blade of the Ruined King's active however should be able to compensate for the loss of burst.
Jarvan IV is one of the best anti-AD-Carry in the game. He has natural high armor from Demacian Standard, a reliable gap closer with a knockup build-in and his ult, which forces enemies to fight him up close. Therefore, building armor items such as Thornmail (returns damage before armor's damage reduction), Randuin's Omen (using the active right after Cataclysm ends will make escaping more difficult), Frozen Heart (best used against multiple Attack Speed reliant champions) and Sunfire Cape (Cataclysm will keep them in its range) can be very beneficial to Jarvan IV.
On the other hand, players might want to use his natural bonus armor to avoid building armor a little longer. With only a Randuin's Omen, Jarvan IV will have 160 Armor, leaving room for other items to increase either his damage or his Magic Resist/Health.
Like all tanky DPS and tanks, Jarvan IV enjoys Warmog's Armor to help him survive while and after initiating a fight.
The Black Cleaver is a must-have on offensive Jarvan IV. Combined with Dragon Strike, Jarvan IV can reduce multiple enemies' armor with up 44.5%, in addition to ignoring a flat 10 Armor.
Zeke's Herald is a perfect item for Jarvan IV later on in the game as it gives the health he needs while giving a bit of damage. The aura also helps synergize with Demacian Standard by giving your allies free ad and lifesteal to encourage them to fight after an initiation.
When facing Jarvan IV, remember that stacking health increases Martial Cadence damage out on you. Although the bonus damage has a cap of 400, it is still quite devastating. Therefore, stacking armor is better.
Jarvan IV has a strong early game. Be careful when playing aggressively when laning against him.
For this reason, wait until his Cataclysm is on cooldown before engaging him.
The ring of Cataclysm can be escaped using virtually any dash in the game. Having one off cooldown can quickly nullify the value of the ring.
Do not underestimate Champion 1v1 potential. His damage output and his Cataclysm make him a dangerous fighter.
Be even more careful of him with his allies about. His abilities have a multitude of utility effects that can let his team win against yours easily even if you personally are able of fighting Champion 1v1.
While Champion knock-up is threatening, do remember that it relies on his Demacian Standard, which is on a 13 second cooldown. If you can get away from the flag, he won't be able to knock you up again.
Scatter around before engaging a team with Jarvan, it will make his ultimate less effective when he can only trap one person.
Take great notice of Champion team. If it consists of heavy AoE (Morgana, Brand, Fiddlesticks, Malphite, etc.) then consider fighting the enemy on your grounds than theirs, or, if possible, eliminate Jarvan IV before the team fight starts to make their ultimates less effective.
Due to the great utility of Jarvan IV, he can participate in a "kill lane" (a lane where the aim is killing the enemy, generally two bruisers against a support/AD carry in bottom lane: popular duo is Jarvan + Leona). If you see such a setup, consider using Alistar or other non-ultimate disruptive supports to stop the kill lane's success.
Using a silence on Jarvan IV will stop him from dashing to you, or even casting his ultimate. You might want to save your silences for that. Even better, sometimes the ultimate is used to do damage and crumble the wall. A silence will prevent him from crumbling the wall, making quick work of him in a bad position.
Attempt and avoid basic attack harassment by Jarvan. Due to his passive, it will almost double his basic attack's damage in early levels, making you generally unable to pay it back and leading to low health.
Jarvan IV has incredible escaping abilities and can jump over nearly any wall with Dragon Strike and Demacian Standard, while buffing his health with Golden Aegis and slowing any chasers. When ambushing him or engaging in the jungle, try to have someone cut off his escape by walking on the other side of the wall. Beware of the knockup of his combo though.
In lane, beware of Dragon StrikeJarvan IV when staying near your minions, as it deals high base damage, has a surprising range and a low cast time, allowing Jarvan IV to farm minions even when defended by an enemy champion.