I think the current best way to stop the obelisk/orb combo with straywood is to give your beastmaster fast and then use force hammer on the obelisk as soon as you are in range. Intercepting creatures will probably shut down this strategy though. You could try using a push effect to move the interceptor(s) out of the zone, them casting force hammer. After that, your beastmaster needs to make a quick getaway so they can summon more creatures. Unfortunately, a complicated trick like this will probably cost like twenty mana.
I think it might need reminding that the advantage with using swarm is sheer number of actions. A good swarm is one that either 1. overwhelms the enemy by sending more creatures after them than can be dealt with at once other than with a zone attack, or 2. Makes a smaller swarm strong enough to still have a good action advantage and buff it. I think swarms should ideally go after the enemy in waves, with the size of the wave depending on the number of attacks the enemy can make against it in one round. Unlike the Necromancer, the beastmaster likes to buff his swarm, and I think he could like it more than the Druid too. I propose that another conjuration like Rajan's fury and Tooth and nail be made, but for giving animal creatures armor +1. The straywood beastmaster should be using a smaller swarm and supporting it with buffs to give them more armor, better attacks and even healing.
This doesn't completely solve the orb/obelisk problem of course. I'm wondering if perhaps the straywood beastmaster could lessen orb/obelisk impact by not summoning their entire swarm right away. Let's do some calculations.
If you summon 6 Bitterwood foxes it costs 30 mana. Add in Rajan's fury/tooth and nail for 7 mana a piece. So now you're at 37 or 44 mana. Way too much to spend as quickly as possible.
But wait a minute, the beastmaster channels 9 mana per round! Why not split it up more over time?
Round 1: (19 mana) cast a mana flower, and ring of beasts (-7)
Round 2: (22 mana) move, cast a Bitterwood fox and a conjuration or zone enchantment that raises their armor; -12 mana (in whatever order is optimal)
After this, use your judgement to decide when to summon more. You need action advantage and buffs to beat the enemy mage. If your opponent has no creatures guarding them, and you have a mana flower, ring of beasts, two foxes and a Rajan's fury (-22 mana out of 39) go ahead and send them in to attack already. Your opponent will have to spend their actions dealing with your two foxes, giving you the chance to summon more of them. And since there's only two of them, your opponent probably won't use a zone attack or orb/obelisk. If your opponent does have a powerful creature, chances are it won't be able to stop both of your foxes from reaching the enemy mage, and if they send their big creature at you instead of your swarm, be ready with a block. (-22 + -4 out of 39) If they have two creatures, you should either summon another level 1 animal, or you could tanglevine one of the enemy bigs (-26 + -5 out of 39.)
If and when they do put down orb/obelisk at least part of your swarm should already be in or adjacent to the enemy mages zone, and therefore between 0 and 2 zones away from either conjuration.