Like Super Monkey Storm, the Thunder Druid is a card most useful against large swarms of bloons. Thus, it should be saved for when it is truly necessary. It's more powerful than a single Spike Storm! for anti-aggro against groups, but much more expensive.
Since it costs 3 less Gold than Super Monkey Storm, it can be played on turn 3 if the player saves all their Gold and is going second.
Unlike other board clears, Thunder Druid has other potential uses after being played, due to it being a Monkey:
Adora can use Ultimate Sacrifice on a Thunder Druid if not planning to re-use it.
Return to Sender or Adora's Salvation can let the player reuse the on play effect, if desperate for another use of the on play effect.
Clever Bloons (e.g. Clever Yellow Bloon) can be played afterwards to trigger their Clever effects.
Unless you're willing to re-use your Thunder Druid with Return to Sender, it is best to replace it. The measly 25 damage is not worth keeping over most Monkey cards, and the opponent could use an Expert Negotiator to their favor.
Reworked animation for Thunder Druid. Now slowly looks at the sky with its wand and strikes lightning from above, instead of aiming at bloons and forking lightning across.
The flavor text for Thunder Druid translates roughly the same in all languages except English. The other versions translate loosely to "Did someone ask for THUNDER?"