As usual, you will need to complete four quests. These four quests are way more difficult than the first set of challenges - you'll need to know exactly where to go and what to do, and it's not always clear from the instructions. After a ton of trial and error, I solved these four quests. So check out my tips below for easy solutions.
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Like at rank 2, you will need to complete four special optional objectives to win Rank 3: Explorer.
Now you'll be able to get (some of the) best upgrades possible for your Nomad Pistol, Larger Utility Belt and Quadruple Thruster Jump, as well as an Advanced Launch Booster and faster refueling. They're all really useful, so unlocking rank 3 as early as possible is pretty important.
Collect living samples of creatures: Kapyane, Jellywaft, Infected Pufferbird
Kapyane is a new creature in the itchy fields. You will find it in the Estate of the Untamed Beast, an arena area at the top of the Mushroom Hill of Si'ned VII. Just tag one of the beasts after dodging the charge attack. When it blinks forward and spawns behind you, turn around and mix it up to stun it.
Infected pufferirds are found in the Gaseaous Underbelly / Festering Chasm inner cave. You'll know they're nearby if there's green gas and a funeral grave. They are slow but they still attack, so you can use a Binding Bile to make them safer.
Jellywafts can be found everywhere, but you can always return to the Jellywaft Pool in the starting area, or go to the pond on the left side of Mushrooms of Si'ned VII.
Hit 2 creatures with Dying Jellywaft Acid.
This one is tricky. Go to the Zor's Planetary Flotsam teleporter - there's a pink Prime Jellywaft with regular green frosts on one of the large rocky islands. Use a bait to lure the Pufferbirds closer to the Pink Jellywaft, then detonate it.
This is really tricky, as you'll need the Jellywaft to be in the right place, but its explosive poison explosion has a fairly wide radius. There are a lot of pufferbirds near the Pink Jelly in this area, so this is one of the most viable places to achieve this.
Kill 3 Amber Pufferbirds with a Bombegranate.
Make your way to the swamps on the way down from the Festering Chasm teleporter - this area is full of Amber Pufferbirds. Use baits to lure them all together, then hit at least three with a Searing Bomb, immediately throw them a Bombegranate to complete the challenge.
Kill 4 blocked creatures within 3 seconds
You can make the same truck to complete this challenge! Use Bait in the descent of the Festering Chasm teleporter swamp to lure four Pufferbirds against each other, then hit them all with Binding Bile. Use a Searing Bomb nearby to clear out any troublesome Amber, then use a Bombegranate to defeat them all.
This one sometimes doesn't trigger, so try to gather at least 5 creatures all in one place. Wait a moment for them to get completely stuck in the Binding Bile, then throw a Bombegranate. In my experience, the nomadic pistol is way too slow.