Hi Cat Ma,
IMHO, the best defence against hypo is hometesting (testing your cat's blood glucose at home).
If you are testing, and getting an idea for how the insulin is working in your cat's body, then the chances of hypo are greatly reduced.
And quite often, if you see that the blood glucose is likely to drop low, it's possible to 'steer' it with food so that the hypo doesn't happen. Sometimes low carb food is sufficient. Sometimes a higher carb food might be needed.
If a cat's blood glucose does drop too low then syrup/karo is the quickest way to raise blood glucose. But it's effect isn't that long lasting. It's an action that may need to be repeated. Or it may be necessary to get food on board also. It depends on the individual situation.
Here is the link to the page on dealing with hypos, which is worth printing out and keeping so that you can have it with your 'hypo kit' (for example; syrup/karo, higher carb 'in gravy' food, and maybe the number of emergency vet services in the event that you might need them. If you do ever find yourself having to deal with a hypo it is much easier if everything is to hand.)
How to treat HYPOS - THEY CAN KILL! Print this Out!!
If you need help, do post here for further advice; or call your vet.
Eliz