Crystalline Crawler has at least three +1/+1 counters on it.
Ornery Tumblewagg does not have summoning sickness and cannot be blocked and killed by an opponent.
Steps
Activate Ornery Tumblewagg's saddle ability by tapping Crystalline Crawler, causing Ornery Tumblewagg to become saddled until end of turn.
At the beginning of your combat phase, Ornery Tumblewagg's first ability triggers, putting a +1/+1 counter on Crystalline Crawler.
Declare Ornery Tumblewagg as an attacker.
Ornery Tumblewagg's second ability triggers, doubling the number of +1/+1 counters on Crystalline Crawler.
Move to your postcombat main phase.
Activate Crystalline Crawler's first ability five times by removing five +1/+1 counters from it, adding ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Activate Aggravated Assault by paying ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) , causing you to untap all creatures you control and to get an additional combat and main phase after this one.
Repeat from step 2.
If Crystalline Crawler begins the combo with four or more +1/+1 counters, this combo will net infinite +1/+1 counters on it and colored mana, which will allow you to target any other creature(s) using Ornery Tumblewagg's triggers.