Pantheon has the ability to engage andis a very powerful ARAM character. He does a lot of quick damage but does not really have any escapes so make sure you engage at the right time.
His ultimate is also a great way to engage if used properly.
During the poke game, keep using Spear Shot to harass. If an enemy mistakenly strays away from their team you can use the combo. Sometimes it can kill them, otherwise will get them very low.
Always start with Aegis of Zeonia, followed by Certain Death and finish with Spear Shot. If you are taking a lot hits you can move back a bit as your CD’s will be back up shortly. Keep using this combo over and over in the same order, the only exception is using Spear Shot alone to poke or finish a low enemy.
Use Heartseeker Strike to engage fights, the way you should aim it is half the circle behind the enemy team if your team is close to or in front of them. It will force them to get closer to your team or to take massive damage from your fall.
Think about which target you want to kill while Heartseeker Strike is casting. Immediately use your combo on them. You can either target a weak ranged enemy or target the enemy closest to your team to be in a safer position.
Start with 2x Spear Shot, 1x Aeigis of Zeonia.
Then get Certain Death once and max out Spear Shot, Aeigis of Zeonia. In this order.
Obviously get Heartseeker Strike whenever available.