1. Basarios hides underground and disguises itself as a boulder/rock. Finding it is not hard especially when it sticks out or is unique on that area (the shape of it specifically.)if you run into it, it'll jump up and damage you. Because of this it's recommended to (if you don't have any bombs) throw a rock or a paintball from a distance. Also enemies jumping at it can wake it up, so kill enemies in the area. The best possible way too damage it while not losing life, just bomb it.
2. Once you have found it, strategy is simple:
When it charges (most used attack), dodge it and use the spare time to hit it. Sometimes you'll see giant white rocks in the area, which if hit by a strong impact explodes. Try to lure the basarios into charging at one of these rocks to deal damage equal to large barrel bombs. Other attacks include a fire ball, poison cloud (it will surround itself with it but the range is only around its body), tail whip, wyvern roar and Basarios Beam (although this might not work most of the time). Always be wary of opportunities to attack Basarios.
Note: If your a bowgunner, then your in luck. There is a fast way to kill it. Bring it above ground. And put it too sleep. Then bomb it. Rinse and repeat till you make go down. The first hit dealt during a monster while it's asleep deals 3x the normal damage!
Use a dragon on the belly, so break off the shell.
Hammer- Just play bull with it and pound its shell. Jump back when it bows its head up (it is about use the poison gas). Just smash it until it dies.
Swords are not recommended due to its rocky skin unless you are hitting its stomach or have esp.
Longswords are also not recommended, because they're long, and have a better chance of hitting it's back then it's belly.
Lances- Aim for its stomach. Be sure to use your guard a lot
Gunlance- Wyvern fire its stomach. Use the same tactic.
 Guide to Bows First of all if this is your first time then bring at least Damiyos Warbow 1 or 2 and bring powers and the optional poison coatings. Bring potions, mega potions, shock traps x3, psychoserum x3 and paintballs.
As for armor just bring anything with skills like High grade earplugs or armor that gives health or defense bonus.
Ok now just get items from the supply box and go find basarios and paintball him. Now stay to his sides and send pierce shots to his stomach to break it and do more damage. Repeat this till he dies.
How to Kill.
A bow is the best way to kill him (If you want to capture it you can still use a bow just make sure you have a lot of tranq bombs) A bow allows you to hit the beast without being hurt by it's poison gas. You also have no trouble dodging it's very slow and predictible charge. Good Luck Hunter , From Draxx
Guide to Light Bowgun
This is probably the easier and faster way to kill a basarios. I'm in the elder cat quests and i use basarios to farm for gold.I usually kill a basarios in 5 mins using shakalala frenzy (300 atack power).
First and the most essencial of all get a light bowgun with 200 attack power, very fast or fast reoad AND your bow must have pierce shot usage or else this guide won't work entirely. You can try to go with normal reload time but probably you will be caught in one of basarios attacks. Buy all the ammo you can use to you Bowgun. You don't need to take potions, mega potions because if you do this perfectly Basarios wont even hit you. Just take herbs they handle the situation.
Next if your fighting Basarios in the volcano take cool drinks or else you will be dead before you kill Basarios. If you are killing him in swamp like the elder cat missions take HOT drinks!!!!! Basarios won't leave the same area and its extreme cold inside. Never forget your antidotes or antidote herbs if you want.
How to kill:
Throw a paintball or paint s shot in Basarios then reload your bow with crag or clust shots if you can use them. If you cant use them just use pellet. Always aim for his belly and head. After you spend all your crag and clust shots use, and this is the main trick, PIERCE SHOTS. This shots have a large effect on Basarios they will make him flinch. Use all your pierce shots and if possible take materials to make more. When you run out of pierce shots Basarios should be pretty damaged and almost ready to capture. Just use normal shots (preference lvl 2 or 3 if you can) and after about 20 shots on the belly it should be or dead or ready for capture.
IFYOU HAVE DRAGON S SHOTS TAKE THEM AND TAKE MATERIALS TO MAKE MORE. THIS WILL MAKE BASARIOS FLINCH, LIMP AND WILL BREAK HIS SKULL BONE AND BELLY.
Dont forget to stay away from Basarios because he uses poison and rarely sleeping gas. Bowguns are better than bows because bow require a lot of stamina and time to kill (i took 30 mins to kill elder cat basarios with bow and 10 mins with light bowgun). One more important thing: keep in your mind that Basarios can do a large fire beam so dont stay on his way its not a very good idea. Besides you can use your bowgun scope so there is no reason to be hitted by Basarios or to get with a beam on the face. I hope this guide helps you. If you have sugestions just write them here.
Have a nice hunt.
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