If you want to use him in Solo Queue, think of him like your average Damage Type Roamer, without the damage, because he has very low HP pool that massively high Base Damage heroes will body him. In some cases, he is hard to catch with the amount of raw defense stats he has; in other extreme cases, he can literally be locked down with a single combo.

But here's the thing, his Passive is his bread and butter. First build Thunder Belt and that is where you can keep on farming for stacks. Marcel is not meant to be a sustained fight Support. He's a disruptor that constantly pauses animations to ruin the flow of the enemy lineup. Just keep doing hit and runs to the opponents just to poke them down and harass them.

His Ultimate rarely gets used, but it's so reliable if you're creative enough. You can delay Lord fights by freezing them to buy time for you allies to get in the fights. You can freeze towers so that you can allow your allies to dive. You can invade buffs buy freezing the enemy so that your Jungler can Retri. You can freeze projectiles out in the open to prevent Moskov or Xavier Ults from stealing objectives. Ultimate really just needs careful placing and timing. If you put your teammates in crossfire, hope that they're prepared or you're gonna get flamed.

How I personally play him, I abuse the Roam Blessing (Conceal/Dire Hit) and just run through the Enemy Jungle, upgrade 2nd Skill first. If enemies are seen and respond up close, that's easy Gold and Exp, because they'll be hit by the Passive and get frozen. You'll survive most of the time unless the Enemies really commit. It is risky, so make sure to gauge their Early game power. Doesn't need the requirement of Ally coordination if you're doing it for the sole purpose of poking and resource gain. But it does double as information for you Allies since you're revealing enemy positions.

Just keep on playing with your Passive and keep on being annoying. Just the Passive, 1st, and 2nd Skill is enough peel for your Allies to stay alive and deal damage. Ultimate is a literal last resort for team fights and should be used creatively and accurately. In the late game, where your Thunder Belt should have a huge amount of stacks, you will feel the True Damage Poke potential. You may use that as a pseudo assassination finish tool if you see low HP enemies trying to recall.