Don't listen to people telling you to switch from Sword and Shield. Definitely experiment with other weapons, but SnS actually a pretty great weapon in this game and you can easily beat Kecha with it. You definitely want to try for one of the major upgrades like others have been saying though, with Seltas being the easy route. The higher sharpness and damage will help a ton.
How are you fighting with the SnS by the way? Your normal combo should be A+A then pull back on the slide pad while pressing A to do your charge attack. You can press X after if you have a clear shot for a nice, high slice, then go back into the combo, or roll out of danger. Pressing X+A too on approach is a good way to cover ground and get in. X+A is secretly one of the best attacks in the game too. If you use it below a low ledge, you will spring up and launch into the air, giving you mounting hits from below the monster. You can also use it to launch yourself off a cliff and you'll be able to land two mounting hits (one from the attack, another on the way down) quite often, making it one of the best mounting weapons.
And you're probably too early in the game to use it much, but the SnS allows the use of items without sheathing. This can be really handy for downing potions, setting traps, placing bombs and especially throwing things like flash bombs.
I'd give this video a quick watch. Even if you think you know the weapon inside and out, you'll probably learn a thing, or two. If you want to hop online later, I can hunt with you for a little bit and you can see how I handle the SnS myself.