As person who fits this criteria (L5 rated with 30.2k HP, full green perks, base damage of 1093), I'll explain why I haven't progressed further up the ratings to L2 or L3, where I probably should be.
The simple answer is I'm using the colosseum to gather 4 cards per cycle. By not progressing up the tower (which determines your colosseum rating), I can gather a 3x multiplier and sustain it long enough to gather 4 cards in 3 sessions. Each session can last about an hour as I can play 30+ hands before I'm out of options - low level common cards. By having a large health pool I can make synergised hands with 500 base damage defeat opponents with 12k HP. This allows me to play casually and still be succesful.
My goal is to collect at least 50% of the legendary cards before increasing my tower rating, which is about +15 cards from where I'm currently at. Besides, the cards you get for completing the tower aren't the best when compared to more recent premium sets.