problem i ran into on stage 20 with similar team was fatman weak hitting and not getting the debuffs stacked. Why 25 was actually faster.

i

Weak on last hit I think is the deal. 

Could be.  I used to run the Skarg, Stag & MM team on L24, which is the same affinity as L20, so it would have been just as prone to weak hits by Skarg.  I must've just not cared. It was my only team option to do the stun/burn technique and farm XP, so I would've run it regardless.     

I do remember switching out Stag Knight for Champfort when I wanted to run L25 for the best possible gear drops....

You are correct. As previously stated, he was my very first AoE Burner. Skarg was also my first Force Lego, so I used him in Arena for a good long while as well. I am very familiar with his mechanics.  

With the team I was using (Roric, Skarg, Scyl, Ursuga & Farakin), there were always more thn 3 debuffs on Skavag before the A2 was dropped, and just like Trips, I do not remeber there being failures to place the HPB back in the day.  Same thing in Arena.  He placed the AoE Burn every time, as long as there were 3 or more buffs/debuff.     

How do I place 10 simultaneously debuffs in spider den