Solar Beam is way too effective against bosses. Maybe cut its base damage in half?
I think there's an element of skill/fun to trying to keep the boss targeted for the full duration of the ability, so making it take a bit longer (via lower damage) seems like a good way to encourage that strategy. Ignoring a readily-increasing swarm of enemies to focus down the boss can make for a compelling risk-reward strategy. Of course, right now it's so easy to out-scale the bosses that there there's only minimal actual risk.
In a pinch, a second lever you could use would be for solar beam to deal less damage to bosses than to enemies, or for it not to apply damage until the move ends. The nuclear option is to intentionally degrade its late-game scaling: Using an over-powered item is fun for a while, so maybe let players have that fun for a while, but no longer. Worse scaling might be enough to encourage players not to use it forever: Having it ignore meta-progression upgrades is a great way to tell the player it is overpowered in the early game but weaker in the late game.