Well randomness will always exist in this games and all games remotely similar to it. Chess is where you want to be without randomness - but the real strategy and skill in MTG when I played(I was a PTQ grinder for a while and managed a shop for a few years - so years of experience as a TO) was from adapting to the near infinite possibilities of what could happen in a game. Mitigating randomness is important - but rolling with it and using what you’re given is just as if not more important.
As far as the exact situation - if you have NO early game plays to contest the mana orbs or beat down on the obelisks I’d just back off. Get out of range of the obelisks. Save the dispel for what he comes at you with. If you have some early game minions I’d slam them out of range of his general to try and kill on of the obelisks - and if he does end up still getting both of the mana orbs and summoning the nimbus that’s what you use your dispel on. I’m not a big fan of using dispels on obelisks early(unless they’re fireblaze, those are way more dangerous), but it can be the right play if your hand can’t handle the 2 damage a turn.
Positioning in these situations is also very important. Lets say you have a silverblade adept and play it - you could play it in a location that would make him chose between using his dervish to kill the minion OR using the dervish to get the mana orb, but not both. I’d say the number one thing newer players lack on as a skill in proper placement of minions - because it’s not something terribly common in other similar games. To a lesser extent it exists in Faeria - but building the board as you go really does limit how many places something CAN be placed and played around.

