You beat the swamp beast's initiative, so you can act before it. You're off the map, since you started 100ft away from the fight, so first you need to move onto the map. As I said, you'll end up in D12 with a single step left, you should be able to move your picture to the square you want to be on. Then you'll have your action. Since you'd still be 25ft away from the swamp beast, you could dash ro get into melee for next round, use a ranged attack, or hold your ground (say by taking the dodge action) and let it come to you.