How can I get it done any techniques or how can I make it look more completed than it is if I run out of time?
Also will I get marked down if I do not use various mediums? I was going to use mostly pencil as it is neater than paint how can I display skills in various mediums?
With paint I'd just build it up in layers, first just do the whole thing in a light wash of paint,then keep building it up. That way if you do run out of time at one of the stages at least there will be no space left uncoloured. I don't really know how you would apply the same idea with pencil though. As for the different mediums, If you really just want to use pencil for the actual poster id say try and show your use of as much mediums as possible in the preparatory sheet. Using different mediums will definitely get you a higher grade though