Elden Ring: Shadow of the Erdtree is home to the most annoying mounted boss I've ever fought, but it can be cheesed thanks to the boss you had to defeat to play the DLC

gamesradar.com:

If you've been diving into Elden Ring's enormous Shadow of the Erdtree DLC, chances are that you've already encountered some tough bosses who've given you a run for your money. Depending on where you've got up to, you might have found a particularly challenging mounted boss with a speedy charge attack and a deadly spear, who, frankly, is an absolute nightmare. Thankfully, there's a really handy way to cheese the fight. 

Before we get into it, be warned that there are minor spoilers ahead for the name and location of the boss in question, so if you don't want to see that, perhaps consider scrolling away now. Otherwise, if you want to know how to kick this thing's butt, stick around. 

Drum roll please, it's Commander Gaius. Found at the back gate of Shadow Keep, this dude does not let up – he rides into battle on an enormous boar which can deal huge damage with its charge attacks, kicks, and tusk swings, and the guy himself has a massive lance and can sling magic attacks to damage you from a distance. Although you can ride around on Torrent during the fight, he's actually slower than the boar, so it's still hard to dodge his moves, and to boot, if you attempt to summon any Spirit Ashes at the start of the fight, you're more than likely going to get charged straight into the fog gate, where you'll get stuck and be doomed to die. 

Reddit is full of threads of players struggling with the boar-riding brute, and if you're one of them, then you might want to try this strategy demonstrated by Twitter user @atmvmta, which makes use of Moghwyn's Sacred Spear and its weapon skill, Bloodboon Ritual. If you're playing the DLC, you should already have access to this as it's obtained by trading the Remembrance dropped by Mohg, Lord of Blood (one of the two big foes who must be defeated to start Shadow of the Erdtree) with Enia at the Roundtable Hold. If you've already used it on something else, you can duplicate the Remembrance at a Walking Mausoleum, or a Remembrance Duplication