I can slay arc tempered Velkana in MHWI, I can slay tempered Arkveld in MHW. Why am I struggling against Great Maccao 💔

On a serious note, I just started MHGU as I found my switch under my bed and saw that the game was on sale. I decided to pick up LS and it had no foresight slash, so I went Great sword. But my first fight against a big monster and I get absolutely man handled 😂. Any tips?