The Jungle:
Sejuani weak early vs other strong counter junglers such as xin, lee sin, udyr. Be aware of this and have your team ready to help you if needed and possibly starting at your other buff to throw off their invade. (most people expect you to start blue)
Sejuani also loses health while clearing the jungle so try running a few potions on her throughout the game.

Build:
This is a full tank build however you can modify it to adjust to a bigger damage role if need be.
For example vs tanks if you don't need the armor you can get Liandry's Torment instead of Iceborne Gauntlet.
(Items with ap and health scale very well on Sejuani because of her ap ratio's as well as health damage ratio's on her Northern Winds)

Don't forget wards!

The Jungle:
Zac weak early vs other strong counter junglers such as xin, lee sin, udyr. Be aware of this and have your team ready to help you if needed and possibly starting at your other buff to throw off their invade. (Zac can start either buff easily)
Zac has great sustain after you gain a few levels.

Build:
This is a full tank build for Zac and the 6th item is up to you based on the situation.

Don't forget wards!

The Jungle:
Jarvan is a strong early champion. He can start either buff is good at counter jungling and stopping invades in his own jungle. However some champions can still kill you in early duels so be aware (xin, lee sin, elise for a few examples).
Jarvans clear will lose you health. So if you like to fight a lot like I do a lifesteal quint or a few potions throughout the game can go a long way.

Build:
This is a pretty aggressive / tank build. If you need more armor and can lose the movement get ninja tabi. If you're behind and need to get tankier early skip brutalizer. The Last slot can be delayed for finishing your black cleaver and can change depending on what your team needs. (I just put thornmail there for the armor since you're already cdr capped with this build)

Don't forget wards!