A grand cross happens when 4 astral items are involved in the following way: A and C are opposite, B and D are opposite, A and B are square, meaning B is square with C, C is square with D and D is square with A. The four items are the extremities of a cross.
Whereas you can get away with a T-square and pay the price, forget it when it comes to grand cross transits. They are the major earthquakes of astrology, the tornados that will rip away everything that is not well fixed and attached. Opposites are about restoring balance where it is missing. A and C put strong pressure via their squares on B and D. And, B and D put strong pressure on A and C in return too.
You can't stop an avalanche, you can't stop a tornado, you can't stop an earthquake! You can't avoid grand crosses. At best, you can prepare for them. Just let them happen, do what you have to do, do your best, go with the flow, don't try to fight or resist them. They are stronger than you. Be patient. It's not all bad, the outcome is always more harmony and balance in our lives.
In natal charts, grand trines are indicative of a strong and energetic personality. It is not necessarily expressed via extrovert attitudes. It can be a calm, quiet, but nevertheless strong presence. Such people don't go unnoticed, whether they like it or not.
In synastry, grand crosses involving items from both parties mean, at best, big challenges to overcome in order to make the relationship work. It may take a lot of adjustments to reach balance and stability. At worst, and especially when there are no other strong supporting aspects, they suggest a high risk of failure since the relationship will be on a permanent roller-coaster. Partners are likely to become seasick and give up, no matter how hard they try to make it work.