Koichiarator is strong, however, you have to note that he has been designed specifically for combat and has a lot of additional features which make him very strong.
- His body is extremely hard and he has very high defenses. Hence he's strong enough to tank those powerful attacks from Goku, Vegeta, and Gohan.
- You missed something out with regard to the beam struggle between Goku-Vegeta and Koichiarator. Goku and Vegeta didn't go all out and only bought Gohan some time to charge up his Kamehameha and strike. It was stated from the start that Gohan wanted Goku and Vegeta to save their stamina to square off against Universe 11.
- With regard to Gohan's Kamehameha,
Koichiarator didn't actually fire a beam attack against him. He is designed to resist strong attacks which is why he managed to survive so long.
- Also, Paparoni used his special abilities to give Koichiarator an attack, speed and defense boost. So in other words, you could compare it to Basil being given the special drug during the Zen Exhibition match.
Now to answer your question with regard to its strength, you can't really gauge or compare its strength and this is why. I want to refer to an incident which happened in Dragon Ball Z where Gohan was training on the Kai's planet with the Z-Sword. We see Gohan struggling to use the Z-sword at the start and to even pull it out. Neither Kibito nor Shin was even strong enough to Pick it up.
However, the Block made of Katchin was strong enough to break the sword into 2. That doesn't imply the block of Katchin is stronger than Gohan.
In terms of strength, as, in actual power, I'd say it's at the level of Super Saiyan 2 Goku at max. (Assuming it didn't have those special features designed to help it in combat).
Some pictures Supporting what I said
