Simple attack sequencing, timers, no coroutines, just state machine with timers

AttackSequencingDemo.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

// @kurtdekker
//
// from Unity forums: https://forum.unity.com/threads/logic-to-create-turns-of-attacks.1137946/
//
// goal: a random attack starts in 2-5 seconds, you have 1 second to defend against it
//
// implementation: simple state machine based on timers
//
// To use:
//	- make an empty scene
//	- add a single GameObject
//	- put this script on it
//	- press PLAY
//	- pay attention to the console
//

public class AttackSequencingDemo : MonoBehaviour
{
	// how long between attacks
	float randomCooldownTimer;

	// if greater than zero, an attack is inbound
	float attackIsUnderwayCountdown;

	// goes up if you defend
	int score;
	// needed to win
	const int ScoreRequired = 5;

	// goes down if you fail to defend, game over at 0 hp
	int lifeHitpoints = 3;

	// it will choose one of these keys to defend
	KeyCode[] possibleDefenseKeys = new KeyCode[]
	{
		KeyCode.A, KeyCode.S, KeyCode.D,
	};
	KeyCode chosenDefenseKey;

	string GameStateDetails()
	{
		return "Score: " + score + ", HP: " + lifeHitpoints;
	}

	void UpdatePossiblyInitiateAttack()
	{
		// handles first and subsequent timer-before-attack-initiates
		if (randomCooldownTimer <= 0)
		{
			Debug.Log( "Get ready for a random attack... incoming...");

			// choose a random time before attack begins
			randomCooldownTimer = Random.Range( 2.0f, 5.0f);
		}

		bool itIsTimeToAttack = false;

		// are we preparing to initiate attack?
		if (randomCooldownTimer > 0)
		{
			randomCooldownTimer -= Time.deltaTime;

			// detects transition from greater than zero to zero or below
			if (randomCooldownTimer <= 0)
			{
				randomCooldownTimer = 0;

				itIsTimeToAttack = true;
			}
		}

		// it is on like Donkey Kong
		if (itIsTimeToAttack)
		{
			// start countdown
			attackIsUnderwayCountdown = 1.0f;

			// choose how to defend
			chosenDefenseKey = possibleDefenseKeys[ Random.Range( 0, possibleDefenseKeys.Length)];

			Debug.Log( "ATTACK BEGINS! Attack pending in 1 second! Press " + chosenDefenseKey + " to defend!!");
		}
	}

	void CheckForSpammedKeys()
	{
		foreach( var key in possibleDefenseKeys)
		{
			if (Input.GetKeyDown( key))
			{
				score--;

				if (score < 0)
				{
					score = 0;
				}

				Debug.Log( "PENALTY FOR SPAMMING! " + GameStateDetails());

				// NOTE: even if you spam, the attack continues!
			}
		}
	}

	void UpdatePlayerResponseToAttacks()
	{
		// an attack countdown is in progress!
		if (attackIsUnderwayCountdown > 0)
		{
			if (Input.GetKeyDown( chosenDefenseKey))
			{
				attackIsUnderwayCountdown = 0;

				score++;

				Debug.Log( "DEFENDED! " + GameStateDetails());

				return;
			}										
			else
			{
				CheckForSpammedKeys();
			}

			attackIsUnderwayCountdown -= Time.deltaTime;

			if (attackIsUnderwayCountdown <= 0)
			{
				attackIsUnderwayCountdown = 0;

				lifeHitpoints--;

				Debug.Log( "YOU GOT HIT!! " + GameStateDetails());

				return;
			}
		}
		else			// you are spamming keys... naughty!
		{
			CheckForSpammedKeys();
		}
	}

	void UpdateHandleWinLose()
	{
		if (lifeHitpoints <= 0)
		{
			Debug.Log( "YOU DIED!");
			Destroy(this);
			return;
		}

		if (score >= ScoreRequired)
		{
			Debug.Log( "YOU WIN!");
			Destroy(this);
			return;
		}
	}

	void Update()
	{
		// only if no attack is pending
		if (attackIsUnderwayCountdown <= 0)
		{
			UpdatePossiblyInitiateAttack();
		}

		UpdatePlayerResponseToAttacks();

		UpdateHandleWinLose();
	}
}