Here's how I'd derive rule of explosion($\bot\vdash\varphi$) from MTP:
- $\;\bot$ --- premise
- $\;\varphi\vee\bot$ --- $\vee$ intro 1
- $\;\bullet\;\bot$ --- assumption
- $\;\bullet\;\bot$ --- reiteration 3
- $\;\neg\bot$ --- $\neg$ intro 3 - 4
- $\;\varphi$ --- MTP 2,5
Now is it fine and would you suggest any other way of derivation?