In order to reach endgame you still have to grind and put hours into this game, I recommend creating a custom difficulty game, select "no wave" and "no zombie respawn", set all zombie HP categories down to zero, select respawn of natural objects etc. Now your game is set up as easy as it gets.

NOTE that every single achievement that is related to finishing the game can be obtained with this method. So you can easily get the multiplayer one as well. Just select multiplayer when hosting a game and choose your save file.

Now to the original point.First of all, disable auto-save. As soon as you reach the final mission which is to go to Cleon's tower, you'll have 2 tasks. To kill the boss and to interact with Veronica who's lying on the floor afterwards. Kill the boss as you'd normally do, make 2 separate manual saves (I recommend 2 because the game is incredibly unpolished and buggy) BEFORE INTERACTING WITH VERONICA. Return to main menu, select start game--> single player--->select a save--->change difficulty---> select legend (ignore the message pop up) load the game--> interact with veronica, BOOM 3 achievements at once.
Hardcore can be obtained separately because at least from my experience, you can't make a hardcore character unless you create a custom game. So repeat the same proccess, only this time keep the difficulty at custom, navigate to the last tab and check the Hardcore box, again, start the game, interact with Veronica and get the achievement.