From what I understand, the Targonian aspects could be as powerful as Sol, but they need hosts to do work in the universe, hosts who can only hold so much power within them.
So the Aspect of the Sun(or solar light maybe? No idea how this works for non-runeterra worlds) could be capable of having a shot at fighting and winning against Sol(but can lose against him too), but Leona cannot as, while she is the aspect of the sun, she only has a fraction of the power.
If this is true, then Zoe, an aspect that can open a way into the universe without a host, would have all of her power. Which means she can fight Sol and have a chance at winning(though with all the stars flying about, I doubt it would be as easy for her to do so as she pretends).
And don't forget, Sol is still shackled by Targon. So even if Zoe is close to his power level, but wouldn't have a chance to win in a fair fight... it isn't a fair fight. With the star crown that is his shackle, every action against Targon that Sol makes costs him dearly. It would be like a Matador fighting a bull. In bull fights, the bull is often weakened in some way(ranging from injuries to things to set off its balance and ability to react), just like how Sol's crown kills off one of his stars(which he refers to his hearts) when he acts against Targon.
If a full on aspect, not one of the hosts, could not take on a foe who stabbed themselves with every attack they brought against them, then that would be rather anti-climatic.
As for why they had to trick him, just listen to Zoe's Vo. Would you want to trust someone like that with an important mission if you could help it? I wouldn't. And as presumably all the other aspects have to work though hosts, they had no one who was strong enough(and reliable enough) to subdue the space dragon through brute force. So they used trickery.
TL;DR
- Injured and shackled, does not have access to his full power and kills part of himself every time he fights Targon.
- Hosts of Aspects that have considerable power, but don't hold a candle to the full power of the Aspects that they host.
Zoe - Full on aspect that has the power to poke herself through dimensions, does not need a host and therefor has her full power. Is childish and irresponsible so she isn't relied upon unless it's necessary
Power levels
Sol ≥ Aspects
Sol > Hosts
Chained Sol > Hosts
Chained Sol ≤ Aspects(with the only aspect known to be able to fight him in person and not through a host being Zoe)
opposed to 