Warning Spoilers ; Please do not read this if you are not far in the game.
Seriously, i don't want to ruin the game for you if you've not played it before and as this deals with the specifics of Lynel fighting i will go into some detail on stuff needed and/or can be helpful for it.
Work on potions, you can get defensive and offensive potions, and even up to level 3 on them.
Learn to perfect dodge or parry as both of these leave lynels wide open or for the best tactic, get ahold of a multi shot bow (2 shots i believe can be found near Rito Town) and just pop it in the face, you can then mount the lynel and hit it a fair bit, my advice is get a heavy weapon one of the two handed ones, the higher the damage on them the better.
If you are struggling defensively you can get a hylian shield really early if you approach the caves beneath the castle, it's on the left side as you approach, you'll know you're in the right spot if you hit the prisons, be careful of the shields guardian it........packs a punch.
Armor wise, you need to go out and farm it, Guardian Armor is really fiddly to get (curse those giant cores) but if you do obtain it, it works excellently in tandem with guardian weapons namely the Guardian Axe++ if not, look into completing the 3 labrynths around the world for barbarian armor and upgrade it to level two it should give attack up (I believe you should be able to do this but as it requires lynel pieces not 100% sure)
Not all Lynels are equal, you will Always have a basic red lynel (the weakest) at Ploymus mountain near zora's domain, this one is pretty good for farming shock arrows as well, practise on it til you learn the patterns of attack, it's somewhat erratic but you'll get it eventually. Lynels out in the field seem to change depending on either how much map is completed or how many beasts are cleared, but the ones in Hyrule Castle are Tier 2 and 3 respectively so it might be worth trying them (they are in guard houses) the Lynel in the colliseum while tricky due to the malic and the mooks hanging around can be good for solo trialing. theres a couple in the baobab highlands just south of the plateau, i'd defo recommend taking the club wielder out as the Lynel Crusher is the best weapon going.
My biggest recommendation, find the Fairy Springs, they are so damn important, theres 4 in hyrule and they will require money to activate but with them you can then upgrade materials and more importantly snag fairies, if you don't have mipha's grace or hell even with it, these are critical, the ones are in Kakariko or Tabantha Plains (you can sight this one from the Tabantha Tower) probably aren't the best for this trick the one under the Gerudo Skeleton in the desert is the best in my books, what you want to do is basically dupe your fairies, gather fairies available at the spring and make sure it's no more than 5, hold 3 of them in your arms and more will spawn, you can get up to 11 this way, which means 11 chances to fight a lynel.
Food wise, 4 Mighty Banana's and a Dragon Horn will give a level 3 attack buff, 5 Hearty Durians will heal you fully and give a lot of yellow hearts (i want to say 20 but can't recall) Dragon Horns you can get from hunting Farosh or Dinraal, theres a spring in Faron known as Riola with a campfire and a good bow (can be found in the lil cave behind the Gerudo Tower) you can farm horns as long as you like (also a valid money source) i say horns because horns i think give the best duration buff to food or potions. Mighty Banana's can be looted from the Yiga Hideout and Mighty Durians grown right next to Faron Tower.
Find Good Armor, If you have amiibo chiefly the link one get Fierce diety armor/sword if not get Barbarian/Guardian armors, Train on the weak ploymus lynel, eat for success make dishes/potions for offense or defense, get the hylian shield from the castle if needed, Get the fairies.
Learn to get really good at initiating flurry rush. This happens when dodging attacks at the last minute. If you need help the shrine on the hill above Kakariko Villiage functions as a tutorial for it. This way you stand a better chance even with low or no armor on.
Save crafting items like Diamonds, Star Fragments, and Dragon and Mini-Boss parts, as the better armors require those as upgrade materials
Take a trip to Hyrule Castle as it has a bunch of high damaging weapons just lying around. You don't even have to fight the enemies in there, just run past them.
Look for plants, fish, bugs etc. that have words like "Hearty" in their names. Cooking these will give you food or potions that grant you extra hearts. There are also food/potions that can increase things like defense and attack power for a limited time.
For armor: Not trying to spoil anything but, obtain one memory and talk to Impa might help you. Also be on the look out for anything on the map that looks like a maze. Certain shrines can also hold armor, so be sure you check all the chests in each shrine you visit.