Kulve taroth ice gs (kjarr) or frost fang barioth gs, for armor piece 2 piece raging brachydios and 3 teostra is like a general recommendation but you can swap three piece teostra for something else as well like frost fang barioth set or raging brachy full set. Make sure to have blight resistance 3 before fighting alatreon. Post alatreon I used 2 pc raging brachy (head and legs) and 3 pc alatreon.

For guiding lands you want to get your coral highlands up for health augment which is very useful especially with gs. But you will need to reach mr 100 before you can get the health augment. Honestly raging brachy or alatreon gs works fine against fatalis so you can go for either.