Ozyton has given you some great advice already. Definitely take that advice and make sure that iceblight doesn't gimp the bow user especially.
I'm gonna tell this stuff to you in no uncertain terms, even if it sounds a bit jerkish, so brace yourself.
First things first, some basic stuff that you probably know, but I'll list anyway:
- Everyone needs to get Health Boost level three if they don't have it. Seriously. Gunners are already wearing cardboard armor and need all the help they can get.
- ABUSE flinch-shot/headbang/knock-the-monster's-face-into-a-wall-mechanic. It works whenever the monster is not enraged, does a ton of damage, and gives you an opening to do a ton more damage.
- Wound parts whenever possible. Unfortunately, your team composition means that only the HBG user can wound parts with a single clutch claw attack, so you'll have to abuse openings to clutch claw downward into Velkhana's face with your bug stick and teach it respect.
Now for the actual fight: the most threatening breath attack, which has killed me on several occasions, is the wide sweeping beam. If the Bow user hasn't skilled for Evade Window, they ought to skill for Evade Window. It's a good skill, and a few points of it lets you dodge right through the beam very easily if you have even decent timing. If the HBG user hasn't set themselves up with Guard+Guard Up+Bowgun Shield Mods, or Evade Window, they need to do one of those for the same reasons. HBG cannot sheathe fast enough to superman dive and avoid these moves; you have to be able to deal with them with the weapon out. If the HBG has a shield mod or two, Guard Up (important!!), and a few points or so in Guard, they can simply stop firing when the beam is coming and their shield will automatically block it for minimal damage. Easy peasy. If they're dodging instead, make sure they are evading in the opposite direction of the beam's sweep, not with the beam, to abuse iframes.
If they're getting hit by the straight-line beam (the one that is often fired twice in a row), they need to stop staring at their reticles (I am guilty of this) and start watching the monster for tells. The straight-shot beams are telegraphed very well and should not present a problem.
If Velkhana flies above them and shoots ice shockwaves straight down after they get knocked down, Ozyton's advice should work. Make sure they understand that they can get a full three extra seconds of invincibility by not getting up immediately after being knocked to the floor. Learning when to stand up is a critical skill in MH, and it's one not many people know about.
With all of that said, if nothing is working and you're willing to completely change your weapon and become a martyr, you can make a build that will allow you to carry them through most fights provided they aren't getting instakilled (and you're not already getting murderized). Equip Sword and Shield. Equip three pieces of Tigrex armor and get three points into Free Meal. This allows you to not consume potions 75% of the time when you use them (yes, you read that correctly). With the other two pieces and your charm/decoration slots, try to get Wide Range level five and Speed Eating level three.
In doing this, you are now the holy grail of support. If you see somebody's health bar so much as twitch, take a Mega Potion. With Speed Eating three, you will have quaffed it in a second or less, and your friends will be receiving its full healing effect, meaning if you're quick on the draw, you can heal them almost fully before they even stand up from getting knocked down. If anyone gets iceblighted, eat a Nulberry. Bam. Problem solved. Also, drink a demondrug, armorskin, and dash juice before the fight starts, and as long as everyone is in the same area, they'll get the buffs, too.
It should be noted that this is an extreme solution to a problem that can and should probably be solved with preparation and practice. You cannot and should not carry someone through Master Rank if they refuse to learn to pull their own weight. The best thing you can do for your friends if you see them carting to the same things over and over is to remind them to pay attention and respect the monster. Everything attack can be evaded, every monster can be beaten.
... Admittedly, some stuff still sucks, like the fact that Velkhana's stupid tail stabs have hitboxes that last long after the stabbing motion has ended, what in Gog's name were you thinking, Capcom, that is some high tier bullfango right there.
I apologize if I sounded like a pretentious piece of dung pod at any point. I have seen too many gunners get nuked because they weren't paying attention to the monster, and thinking about it gives me war flashbacks.
Good luck.