Today we played champion and we don't get it... It is an advantage if they take out your champion afte he/she has attacked as you can get a new champion inmediately and attack with him and get the points again. There is really no incentive in killing the opposite champion, actually there is little incentive to attack someone else with other than the champion, specially in the 2d-3d turns when your opponent still has 2 reserves in case you manage to injure 3 enemies.
And about the VIP challenge, it is like you can chainkill enemies easily and get a lot of points in your turn, I kill the VIP, I call VIP on other enemy I can kill this turn... it is ridiculous.
What do you think about these two challenges?

They both require different tactics and approaches other than the brawl mentality. Others attacking in champion isn't useless, unless you have brawl mentality that says "if it's not void, it's a failed attack". With that in mind, then other characters attacking doesn't matter. But, in champion, water results are one of your best friends. You use them to bring targets to the champion. Board positioning and maneuvering the enemy is a very important part of that scenario that doesn't exist in brawl.

Assassination is similar, in that, if you come at it with a brawl mentality, you will not do well. In assassination, you need to protect certain targets, but you also need to have some foresight. In brawl, you can scatter your ninjas like ants after their kill was kicked over and go after whatever looks easy in the moment. In assassination, you have to look at all your ninja as the next potential target. keeping tighter formations so you can fill in gaps, and run interference on the next potential target at any given time is important. Breaking formation just to attack some rando will help your opponent far more than it will help you. And, assassination is one of those scenarios that could be very high scoring in general. Some scenarios end up with higher scores than other scenarios just naturally.

Sure, I chose water before my champion had attacked, but did not see the purpose of doing after that knowing initiative is almost a 50/50 random thing. The problem I really have is with the "inmediately choose another champion" after he is injured. I can go kamikaze because my opponent maybe won't attack him, because if he injures him I will choose another champion and get points again. He could try to impede it with air, but most times isnt enough to prevent not being attacked next turn.
I think killing opposite champion should be rewarded too, now the reward is your champion bing killed.

For assassination, how about a randomizer that determined which of the ninja is the next target rather than being able to choose the weakest and most available target? If it is a 6 ninja mission, a d6 would do perfectly. Oh wait, the game already comes with those! Assign an element to each of your ninja and roll to determine the next target.

If you wanted to change it further, you could move this roll to the end of the round in the cleanup phase. That way, there would only be 1 assassination target maximum per round. That would REALLY slow down the action after the assassination was completed though. . .