I don't know whether this fits a mod or a scenario, but it changes gameplay. So let's start.

My idea is having some kind of a gungame. You may have seen gungames in other games.
But how can you possibly have a gungame? By switching units. The specific order isn't done yet, but the idea is that when you kill an enemy unit, your unit automatically upgrades. The units to be used are non unique ones, going through barracks, archery range, stable, and siege workshop. The last kill in gungames is usually a knife kill, so, to remain faithful to the style, the last unit will be a Spanish villager (with Supremacy).

Example: you start with a militia. Kill one opponent, you get man at arms. When you get to champion, and kill someone, you change to archer. Kill someone, crossbowman. And so on, changing between all units previously mentioned.

If it cannot be done with triggers, I'll also need help or tips to mod.

Your idea is possible by triggers in some way, but overall I think when a player who lead at begin will have huge chance to win too at end too (it will be harder to balance than actually making the map).

And about guns

Are you talking about Arms Race known from Counter Strike?

it's original called Gun Game
arms race is just a new stupid name VALVe made up