These elixirs are available in the crown shop (and as a reward from KI games and Grub Guardian), and boost +120 for critical /critical block. The problem is with the decline, they don't add much for our max level wizards, making them useless. At level 50, they add about 25% increase; while at level 100, only about 10%. An easy solution is to change them both to add a percentage (like 25% more) instead of a number. Or KI can add a boost on them so they don't get under powered by the critical decline.
Re: Change needed in Vengeful / Fortification elixir
I'd never thought of this because I never buy them (thank you Grub Guardian!) but I think you're right that the boost would make more sense as a percentage. These elixirs are probably more desired at higher levels and for anyone paying crowns for them the boost dropping at the levels they could be most tempted to buy them seems a little odd.