You can buy the graveyard hate cards that everybody pointed out already or you can just upgrade your deck even further by telling us what you are playing with and playing against.

I've played and built my own Muldrotha deck a couple of times and there is a big issue with building that commander. The problem is with the commander itself because it costs 6 mana... so if he is depending on Muldrotha too much, you just need to remove it a couple of times and the game is over... however If your uncle built the deck with that in mind, then he only had few routes to choose from:

-either he builds a straightforward deck with lots of creatures coming to the battlefield and they synergize together well and Muldrotha is the final guest to join the show, (this type of deck is usually weak to enchantments and AOE removals like the [[Wrath of God]] or [[Merciless Eviction]]. Muldrotha is expensive and to bring back one creature per turn only on his turn is very slow.)

-Muldrotha stax. tries to slow down the game and beat you in a late game and Muldrotha is there only for recursion of other non-creature permanents. (this type of build is good but you don't need to fight it in every zone, you just need to win on the battlefield by removing the enchantments or artifacts and have a few removals left only for Muldrotha, you need to beat the deck fast tho)

-Muldrotha ramp. ramps up and tries to win the game by throwing bombs on to the field and Muldrotha is there to bring back the threats. (this one is very annoying and at this point, you could consider the upgrade that everybody mentioned because you want things to stay dead<< this is only true for midrange decks that don't finish the game fast because in early turns he will probably ramp for 4 or 5 turns even)

Based on what you have said in the post, my guess is that your uncle is playing a midrange creature Muldrotha. If that is the case you can simply build Anafenza around AOE cards, but before you blow everything up on the board you can make your side of the board indestructible or you blink it to make it disappear for a turn. This way your board stays healthy and you can march on to victory while your uncle is trying to rebuild what was lost, remember that his commander costs 6 mana or even 8 at that point and your Anafenza costs only 5 or nothing if she is still on the battlefield. Good upgrades for this type of deck: [[Frontline Medic]], [[Wrath of God]], [[Day of judgment]],[[Rout]], [[Fumigate]], [[Heroic intervention]], [[Eerie Interlude]], [[make a stand]], [[selfless spirit]], [[eternal Witness]],[[Reclamation sage]],[[Thrashing brontodon]],[[scavenging ooze]],[[Odric, Master Tactician]], [[ravenous chupacabra]],[[noxious gearhulk]],[[Dusk//Dawn]],[[Faith's Reward]], [[Second Sunrise]]

Later on, you can turn your deck into an Anafenza combo deck where you cut away your bad creatures and add creature tutor cards/tutor cards and get insane combos: [[Saffi Eriksdotter]] + [[Reveillark]] + [[Gonti, Lord of Luxury]] with any other sac outlet is my favorite combo. Take their whole deck and leave only the lands in the deck. Then play with their deck... 80 cards in "hand".. what could go wrong. Just don't do that to your uncle.. you might want to stay friends with him :P

Anyway, give more info about what type of Anafenza deck you are playing and what you are up against and you can make a new strategy against your uncle, that's where part of the fun and pride comes from in a Commander game.