Heal Zir’an is also insanely OP when I’m playing Burn Songhai.
Jokes aside, Magmar has excellent tools to deal with Swarm strategies in Makantor Warbeast and Plasma Storm, making them a natural counter to your deck. It’s suggest playing a different archetype as well so you don’t base your perception of Magmar solely on an uphill match up.
I believe you’ll find that Starhorn and Vaath are middle of the pack at the moment while Ragnora hasn’t really hit her stride yet.
That aside, there probably are adjustments you could make to your deck to shore up the Magmar match up, but knowing your current deck list might come in handy there.
Now, a few words about Makantor Warbeast. It is the faction’s dominant staple but it also is a six mana minion. As great as it is at clearing a bunch of 1/1 or 2/2 Wraithlings, do consider that the average minion in that price range is not a 1/1 or 2/2 Wraithling; it isn’t the unconditional board wipe you seem to think it is considering that you’ll face many a minion that’s x/5 or greater at that stage of the game. This isn’t to say that MWB isn’t exceedingly powerful. It is, which is why it makes it into every Magmar list. Still, your are evaluating it solely based on being countered by it.
And lastly, cards like MWB, Plasma Storm, Frostburn and Tempest exist for a reason. They are designed to counter swarms, so the bottom line is, the card is working as intended.
Now, personally, I’d probably play some aggro Cassy instead of swarm. Play it well and you’ll see that Abyssian can do very well. But you already ruled out the possibility that the issue at hand stems from you or your deck in your opening post so I guess giving advice is just wasted effort at this point, right?