What i did to get better at shielding and timing is this:
Go into a brawl with 1 level 9
And don't attack
approach, and when they attack, just shield
and see how long you can last.
When you think you last long enough with one cpu, add another lvl 9
then another lvl 9
After that, go back with 1 lvl 9, and teach yourself to shield, then punish
whether it'd be shield grab, up smash out of shield, or anything like that
CPU's are never as good as players, so just keep trying
And luckily with brawl, when you think you got it good enough with CPU's, try the same thing on Wi Fi.
Just go into a brawl and shield the entire time, approach players offensively, and just shield when they attack. If its a timed match, see if you can keep 0-50% with no deaths the entire game.
It takes time, but it'll definitely help your play