It is often best to choose the most expensive card, to take advantage of the large shield.
Note that the opponent can see your picked card cost by calculating the increased shield in groups of 15, which could give away some of your selected cards.
Any +150 or more shield (10+ Gold) may suggest a control opponent, or a midrange combo deck if suspecting Double Trouble.
An opponent with +45 shield (3 Gold) could potentially suggest picking another Nature's Clarity.
+30 shield (2 Gold) could suggest The Eternal, as this is the only non-Hero card of this cost that stands out in non-aggro decks. It could also represent Bloon Melter, which is commonly run alongside the Eternal.
At best, Nature's Clarity grants +270 shield. The card with the highest Gold cost is the Sun Temple, at 18 Gold.
This card is best on control decks, because exchanging large shield for cheap cards is not worth it over choosing an aggro-friendly hero such as Quincy.
Be careful about opponents who use Shields Down!; this card neutralize all Shield gains since being played.
This card existed in the data of Bloons Card Storm since the Firstlook test, though it went unused until the full release. While it was always an Obyn-exclusive Power card, it originally had different properties.