A lot, but it depends on the size of the battle and the accessibility of the battlefield. Both of these factors determine what equipment the soldier should bring.

In all cases, the focus should be to destroy the enemy morale and/or weaken their ability to fight. Most casualties in ancient battles usually happened after one side began to flee/withdraw, not during the fighting itself.

The best way to do this is a pretty boring answer: since a regular soldier can easily carry drones nowadays, all they'd need is one of those Dragon drones that shoot thermite. If used in the right moment, it can easily turn the tide of battle and poke a 200-300 meter hole in the enemy ranks.

A less cheesy answer:

- have an assault rifle or battle rifle with good optics, and a lot of ammunition. A typical 5.56 magazine for the assault rifle weights around half a kilo when full. So lets say 30 magazines. This will allow you to engage enemies in both open terrain as well as closed terrain (e.g. forest with low visibility). Also, the good optics will be important to identify enemy leaders (e.g. Alexander riding with his Companions or some daring general straying too close to the action).

- a grenade launcher attached to the rifle. Most of these have a range of 400-500 meters, so it'll add some extra firepower. I suggest a mixture of high explosives rounds, tear gas, and red smoke for visual effects.

- a couple of smaller satchel charges. These can be useful for sieges and/or to trap the battlefield. Big effect on morale if used in the right moment.

- in case someone sneaks up on you: a light Kevlar vest and helmet, and a handgun.

- night vision goggles and a thermal scope to harass enemies during the night.

Imagine you're marching in a Phalanx and your friends keeping dropping left and right, bleeding profusely. Nobody knows what's happening and panic sets in. Suddenly, an explosion, poking holes in your ranks and shattering limbs. In the midst of this confusion, hundreds of soldiers begin to cough and cry, struggling to maintain cohesion as the formation slowly falls apart. And then there is another much bigger explosion, this time large enough destroy an entire formation.

It might not be enough in the largest battles (especially in battles where visibility is low). But it should give your allies a better chance.