YMMV / 20 Minutes Till Dawn

• Game-Breaker:
  • The Frost Mage upgrade tree has Frostbite, which removed 15% of an enemy's max HP when they are frozen. Combine it with upgrades like Sight Magic (enemies in vision range have a chance of receiving elemental effects) and Splinter (defeated enemies break into multiple bullets that deal a small fraction of your normal damage - but this does not affect elemental damage whatsoever) and you can apply a huge chunk of damage to enemies over and over easily. This also does not even go into Freezing's incredible crowd control capability; with these upgrades, you can easily have a ring of frozen, helpless enemies around an island of safe space to move around in, and with a sufficiently fast-firing weapon, you can essentially keep freezing bosses, stopping them mid-charge, keeping them from attacking nearly as frequently as they could have, and even freezing their minions and adding an extra source of damage.
  • The Version 0.9.0 update turned Summons and Summon Builds into this. From being a gimmicky build to a small addition to a mostly elemental damage focused build, Summon Builds quickly became a new meta for how vastly powerful they became with new additions such as the Holy Spears helping tanky builds with increased damage per remaining heart, the dedicated trees for increasing summon attack damage and speed, and reworking how many of the summons like the Ghost and the Scythe worked while removing some less reliable options like the Magic Knives, making them much faster, deadlier, and effective. Among the changes included granting Summon projectiles on-hit effects or granting them elemental effects themselves, and the damage and quantity of attacks they can output can get so high, you could quite conceivably end up gathering so much steam for your summons that you can do nothing, let your summons do the work, and still win. Add a new healing system in the Soul Hearts, and you can quickly make yourself all but invulnerable even if you did get hit while you idled, too.
  • Dasher's Deer Form is invincible, has increased move speed, and deals Collision Damage based on summon damage and movement speed. It activates every 10 seconds, which gives you plenty of time to mow down the enemy hordes. She was even more of a game breaker on release, where the condition for Deer Form was killing 25 enemies.
  • The Soul Conversion upgrade converted a Soul Heart into a max HP every minute. In addition to making runs much more survivable, it synergized incredibly well with other upgrades that scaled off of HP. The 1.0 release would replace it with Soul Expand, which merely increases your Soul Heart cap by two.
  • Lillith and the Bat Gun. Combined with the right upgrades, you can literally stand still and hold down the fire button and watch as the screen disappears under a flood of enemy-seeking missiles.
• That One Attack: Shub-Niggurath's charge attack can be quite tricky to dodge, especially in the heat of combat, due to moving much faster than any other attack in the game and it not being obvious when they've stopped tracking you. This makes Shub-Niggurath one of the most dangerous foes in the game. It's especially bad starting at Darkness 6, where bosses do double damage.