Well i did fairly well at evo and I'm telling you right now that having a good invincible assist was (and still is) more than half the reason why i made it as far as i did (and have me good defense) If you look at top 8 of evo 6 of the 8 teams iirc had invincible assists. Both of the teams that i lost to had invincible assists, the guy that i beat, to make it out of pools, had invincible assists....
And the best skullgirls player in the world has not 1... But 2 invincible assists on his team. If you want to up your defensive ability, invincibles are the shortest path to an increase in defensive ability. Other than that you will have to get good at jumping backward, dashing backward, pushblocking and reading your opponents habits so that you can make the right guess on defense should they make you block something close range. But honestly, defense in skullgirls is mostly about not getting hit or blocking the opponent in the first place. If you do happen to block, then mixing up between pushblocking out, or mashing reversal invincible assist is a good way of defending. Also, if you happen to block a lockdown assist, more often than not your opponent will try to run a mixup on you while you are blocking... So doing a reversal pbgc there or simply a reversal just as you come out of blockstun... Is also really good for defense.
But like i said, not being in a bad situation in the first place is the best answer, and invincible assists help out for that more than anything else.
If not, then you just have to make great use of the things i already mentioned such as backdash, jump back, long ranged pokes like air cymbal, pushblock, pushblock autoguard, and pushblock guard cancel and reversals in general. Also last but not least, blocking in sg is really, really, HARD. Everyone gets blown up, even duckator. But the point to know is your first defensive mistake is generally being put into a position where you have to block, in the first place.