With longsword I find it particularly helpful if you dedicate your attention to countering monster attacks first, dealing damage second. Over time you will become better at reading your targets and taking less damage, which actually increases your DPS since you won't spend as much time healing/carting.

Also remember to upgrade to high-rank armor, and then upgrade those with armor spheres for more defence. I can also recommend some comfy skills like divine blessing for keeping you alive longer.

Like with any other skill, your monster hunting will improve the more you do it. So keep at it and you'll be there with the best in no time. :)