maybe to start off with mecha-godzillas combo, try using rockets until you reach 50 stamina left, unless if they block use it until 75 to stall them, as they keep blocking keep walking towards them. when your reach them and the 75 stamina mark use grab then continue the combo, LMB1 + LMB2 + SP1 + BEAM + SP2 + ROCKETS = 383 DMG.
More OP tips: For orga, in between one of the attacks, just straight up run into the enemy. It does damage naturally, it gives you time for specials to cooldown and gain rage, yeah you thought they removed the old sp2 charge? Nah just run into your enemy as orga. For MG21, try squeezing missiles in between combos, especially during ULT combos. It adds a lot of damage.
For siren you could get an extra 100 DMG since its beam can be used mid combo and id recommended using it after the rock or before it and here is why Before: can drain opponents stanima so the rock can be more effect After: they legit cant see or hear you use your beam when using it after the rock and the only way to counter is by moving backwards so the beam cant reach (unless your like me and have terrible aim allowing the opponent to see the beam and block it) also i dont recommended moving forward when beaming as the rock makes siren head turn away from the opponent and cant look back untill the rock hitbox vanishes
i wouldn't quite recommend zillas ult combo in just about any fight because it's arguably the most predictable and easy to counter ult in the entire game so unless you spend your special 3 to drain their stamina then your more often then not getting the entire combo destroyed because they blocked it.