Well you have 7 slots you can fill, so 4 of those slots for instance, you could fill with:

1. Lighting res +2

2. Fire res +2

3. Water res +2

4. Wind res +2

Do this for all 10 tools, and you have +20 for the four elements, right?

@Light, 10 tools, each can give potentially up to 10*5*2%=100% total res to all (you cant have 2x the same stat and we are considering only max 2% res as of current tools, however the ones you buy for ingots can roll old 4% stats). He has a total of +92% res out of 100% (total of 72% is shown because of -20% earth element weakness) possible, which is perfectly fine. Interesting build, whether it's worth sacrificing so much of base stats (basically 400~of each)- it's debatable.

@edit, my mistake, there's indeed no strong element. Corrected that, sorry for misleading.

your pretty close.

you can only have 1 of each type of elemental resistance per tool. so on each of the 10 ninja tools you get 2% lightning, fire, water, earth, wind resistance which means a 20% total for each element, -20% for the one you are weak against makes that balance out to 0%.

you get the 2% elemental resistances from the ninja tools that drop from ti, not from essence, older tools used to have up to 4% resistance in an element, then after farming them you just need to reforge the stat onto your current gear.