If I get attacked with a 100/100 directly, can I make my planeswalker take all the damage?
No, the attacking player chooses what player or planeswalker his creature is attacking when he/she declares an attack.
And in terms of redirecting damage to a planeswalker which is what you may be confusing this with.
Only NON combat damage may be redirected to a planeswalker. And that damage can only be redirected by the controller of the damage/ability, not the player receiving the damage.
So I can {lightning bolt} and target you. Note I still can't target walkers with this spell. However, when bolt is going to resolve, (meaning both of us have passed priority and nothing is gonna stop it from resolving) I now have the opportunity to redirect it to {dack fayden}. You however cannot redirect it to him since it's not your spell. And it doesn't work the other way. Life gained cannot be redirected to walkers as sweet as that would be.