BARBARA BLOOM IS SO BROKEN!
EDIT: Miyoho has announced that Barbara's ICD is a bug right now and will fix it next patch on v3.1
When you cast the ES of Barbara and inflicted to an enemy which is already affected by Dendro, it will produce a Dendro Core. If another target was detected by this, another Dendro Core will produce. And because only 5 Dendro Core can co-exist at the same time, producing more than 5 Dendro Cores will cause the oldest one to explode immediately, dealing AoE Dmg.
On top of that, Barbara's ES has an ICD rule (3 hit/2.5 second), and for this one, it is using a hit counter. When the elemental skill of Barbara touches any of the Dendro Core, it considered as a hit, every 3 hits, it will apply Hydro. This will cause massive hydro application. If you have massive hydro and dendro application on an enemy, you will produce Bloom reaction which is a Dendro Core.
With this mechanism, we get to see Dendro cores explode so fast and then we see how fast it produces Dendro Cores because of Barbara's ES. It's more like a chain reaction whenever Barbara's ES detect a new target with Dendro on it. This is good for dealing multiple enemies but not so much for dealing a single target.