Petrify is an elemental combo in Dungeon Defenders 2. It is applied by dealing magical earth damage to poisoned enemies.
Petrified enemies are stunned and receive 5% more damage. However, petrified enemies don't lose health until the petrify condition ends. This effectively stacks damage onto the target and applies all of the damage the target receives at once, making this a great combo for clogging up lanes.
This is the easiest way to apply the petrify combo, being available at the start of the game and requiring no MODs. Use the Poison Dart Tower to poison enemies; then, strike them with an Earthshatter.
Despite being an easy way to access the petrify combo, this requires at least 100 Defense Units to build one PDT and Earthshatter.
The PDT + Earthshatter combo is great for tackling on minibosses, but can get mowed down by hordes of enemies. Using an Earth Toss shard will slightly increase the AOE radius of the earthshatter's spikes.
If you're looking to use towers to apply petrify, but you need to do so to a large group of enemies, consider using a Skeletal Ramster with a poison servo instead of a PDT - or, if you have enough DU left, you can use both! Despite how great their attacks are at AOE, ramsters have poor range and cost 10 more DU than a PDT. Don't forget to bear those things in mind if you decide to switch!
Shattering Torpedo Reflect Beams
The Shattering Torpedo shard causes Reflect Beams to deal crushing earth damage - all without the use of a MOD! Any defense can be used to poison enemies. The PDT is a decent and fairly cheap tower for applying poison, but you may want to use a defense that is better at AOE damage, like the Skeletal Ramster or Weapon Manufacturer. The reflect beams themselves are also cheap too, and their torpedoes can hit enemies from a fair distance away. They can even provide protection from any monsters wanting to poke your defenses from afar.
While this combo is relatively cheap in terms of DU, it requires a chaos 8 shard, which can only be obtained from onslaught floor 30+, prime incursions, or the Gran Ma'ster. Cyborks can also disable your reflect beams, and depending on what setup and shards/mods you're using, berserkers can charge through your defenses. Vicious Strikes can help deal with these 2 enemies, but you can only obtain this shard by obtaining 155 stars in Mastery (you will eventually be able to obtain a second one in Prime Incursions).
If Vicious Strikes isn't available, you can use cannons or another single-targeting defense to handle berserker leaks. It's rare to end up with more than 2-3 berserkers leaking at a time, so you shouldn't need any AOE towers.
Petrify or Electrocute?
Both elemental combos are great at providing crowd control to enemy lanes, and are very similar to each other. However, there's a few key differences:
Petrified enemies receive 5% more damage, and the status effect used to apply petrify is poison. This is great for adding insult to injury and dealing extra damage.
The status effect used to apply electrocute is drench, which can aid at crowd control even further. If you aren't able to apply storm damage to electrocute an enemy, you might at least end up with them being slowed.
What this essentially boils down to is personal preference. If you need more crowd control, use electrocute. However, if you doing just fine in the crowd control department, petrify can help you kill enemies faster. It's recommended to start with petrify, and determine from there if you need to switch to electrocute.