With this type of question the answer really depends on the construction of the rest of the robot.

For example Tiny Toon has been constructed with a steel armour box that sits on the base plate as a result the base can be made out of relatively weak material. All it has to do is support the weight and hold all the pieces in position, the impact of any attack is carried round the welded box.
My robot Tron has a similar construction and has a base plate made from 6mm polycarbonate which is cracked! but the box that fits over this is 5mm welded steel sides and 3mm steel top.

Looking at your machine it looks like the armour parts are bolted direct to the base plate and so a lot of impact stress is transferred directly to the base plate, hence it's failure.

If you have the weight you may want to consider Hardox steel or equivalent, if not I would go for 12mm UHMWPE, this is the same material used on the wheels of Saint HW.
I would also consider some method of tying the armour together with steel plates.

Hope this helps

Craig