The most effective way to deal with Magmar is to complain about them on the forums with the knowledge that they will be hit by a nerf next patch. This will help with the frustration, since the frustrating cards will most likely be nerfed really soon.
After that, what decks do you play? What rank are you? High tier Magmar decks rarely run no hard removal outside of thumping wave, so answer or die is a good way to deal with them. Against mid range Vaath, anything hard to kill with damage works well. Things like Spellhai, Obelisk Vet (most of them do not run plasma storm at higher ranks), and Lyonar can play threats that are hard for Vaath to smack. Against Aggro Starhorn, other Aggro decks work really well. Spellhai, Obelisk Vet, and Tempo Lyonar can all out-speed it.
As for general tips, play around Makantor when near 6 mana. Position in an oblique line and try to prevent Makantor Value. Try to get the Magmar’s health low in the earlygame. Vaath needs it to gain value from BBS and Starhorn uses a lot of self hard cards as well. Remember that Vaath’s BBS can be dispelled, so sunbloom, ephemeral shroud, and chromatic cold can shut it down. Entropic Gaze is absurd. The best way to play around it is to complain about it and hope the Developers slam it with the nerf hammer.