The hardest achievement in the game. For this achievement, you’ll need to play on the hard difficulty on every level, ensure 80% accuracy and also complete all bonus objectives. You must also get all 5 stars within 1 round, meaning that you can't do the bonus objective and hard difficulty on different playthroughs.
As recommended in the roadmap, it'll be easier if you play on a lower difficulty, farm gold and revisit levels with a lot of upgrades. With be projectiles, spells, heroes and rooms, you'll find a lot of the objectives a lot easier.
In castle vs. castle levels, the tactic described in "Against the Machine" will work great, and is extremely efficient. It is often a good tactic to try and capture the flag first, but also damage the castle as a back-up. Remember that you should have Guardian or Ghost Army equipped to quickly help kill a large amount of enemies at once.
Also, if you are struggling on any levels in particular, you can find a short guide for some of the harder levels HERE.
Once you obtain all 300 stars available in the game, you will unlock the achievement. You can use the leaderboard or the campaign selection screen to see your current progress.