If you wanna kill him easily, Equip the dual blades and power them up with R and i think the default for them should be paralyze (which stuns lagombi for a LONG time, enough to get a ton of hits in) Its the most suited to the hack and slash gameplay and has a good roll dodge that can be used pretty much off every combo.
Check the blue box before you head out and grab the paralyze trap, then when he dashes, lure him into it. Stuns him for almost 10 seconds which in MH is a TON of time to get free hits in with the big weapons.
- When he taunts is a good time to get a few big hits in (when he stands back on 2 feet exposing his stomach) be quick and get in and grab a hit or 2.
- When he digs into the ground, he's about to throw a snowball (dodge left or right to avoid it and if you're quick you can get another 1-2 hits in after)
- His 2 most anoying attacks are the dash and the "sweep", The dash is pretty straight forward to dodge and he usually does 2-3 dashes in a row if you stay relatively close to him, dont bother trying to get a hit in here and its risky to try doing it straight after coz he usually follows it up with a snowball.
- The sweep move is the one where he spins on the spot, VERY hard to dodge and see coming but if you do manage to dodge it, he usually leaves you time to get maybe 1 heavy hit in before he moves.
If you can handle them, heavy weapons are very good here. If you manage to get 2 hits with the great sword or hammer then it will almost always cause him to stumble, giving you time to get another 1-2 hits in.
Best advice i can give is have patience, you wont kill him in 1 assault. Have patience and learn his attacks (which iv explained) and dont go for too much, with almost all the weapons, you are only gonna get a maximum of 3 hits in before you'll need to dodge or back off to wait for the next opening. I wouldnt bother with a ranged weapon for this guy, he's too fast and they are too weak to make him stumble. I recommend Great sword/long sword/dual blades/hammer.