It did'nt work for lvl 2 because you forgot to replace de "level: 0" with "(Level of Incineration for (Triggering unit)) - 1". level 0 means the first level on the ability, i know it's weird but programming languages are 0-based, so 0 is always the beginning, so when your ability is level 2, then the field level you want to change is "level 1" hance "(Level of Incineration for (Triggering unit))", which will be 2, minus 1 resulting in 1.
Edit: for the last level i think you will need to decrease and increase the ability level, instead of increasing and decreasing, since it's the last level. It's just one conditional for the last level. Also whenever you want to do a calculatiion in gui, use the Math - Aritimethic, and be aware of the parenthesis.