Two rounds of that & I hit my urgent which to my surprise was not Deviljho.
Surprise! Deviljho is not the toughest monster.
When it's on the ground, its weaknesses are water and ice. When it takes flight, its weaknesses are fire and dragon. It's difficult to hit it when it's flying so a water or ice weapon would be better than a fire one. Though most people use a dragon weapon on it, because they have the highest attack, and dragon still some some slight extra damage.
Technically, its weaknesses are paralysis and sleep. However, it isn't worth it to use a regular weapon of this type, since they don't have very high attack. Instead, this works only for a gunner, preferably two gunners, who will do two things: paralyze it so the others can hack away at it; and put it to sleep, then have everyone lay two Barrel Bomb L+ by its head. Everyone should bring ingredients for more Barrel Bombs, as well as material to make ammo to give to the gunners, as it can be sleeped/paralyzed at least five times usually.
Unfortunately I don't think anyone in our group has the equipment to gun this one. I know a few gunners myself, but I don't know if they would be willing to help us.
So fire is the attack to watch out for?
One of many. It also has a dragon attack it uses regularly, and Rathalos armour is weak in that area so watch out. When it takes flight, it will use thunder and ice attacks. The ice ones are pretty easy to dodge since you can see them coming, but the thunder one is pretty quick so be careful.
It has fire resistance naturally and I think it has guard +2. Which pretty much makes all attacks not effective.
That's what Guard Boost does. Guard +2 lowers knock-back and reduces stamina depletion, but it does not increase the number of attacks which can be blocked.