Here is the full list of all 12 Happy Summer Holiday achievements.
Killed by the Weakest Monster.
Use weapon to slay more than two monsters.
Escape Ending Reached
Stayed in the bunker for more than 1 minute.
Flight time greater than 60 seconds.
Acquire All Firearms Weapons
Into the Eve of the Dawn War.
Acquired after one pass
Kill the final boss
Dive time exceeds 30 seconds.
3 minutes with the monster after the monster appears.
Trigger UFO Ending