Strengths
Heroes that Maecel counters. Marcel's second skill allows him to quickly close the distance to enemies. Heroes without mobility can easily get caught by him and be controlled continuously. Examples include Xavier, Layla and Vexana.
Weaknesses
Marcel's ultimate is not unstoppable. There are some heroes who are not affected by his ultimate's control. Heroes who can continue dealing damage during the duration of his ultimate can effectively counter Maecel. Examples include Odette, Ling and X-Borg.
Tank ltems
Since Marcel's damage and shield both scale with his defense and HP, he is better suited for tank items. This is the build I recommend. Thunder Belt and Oracle are his core items. Magic Boots provide cooldown reduction, Thunder Belt slows enemies and gives hybrid defense, Oracle increases shield effects, Dominance Ice reduces the enemy's healing and shields, Antique Cuirass lowers the physical damage you take, and Guardian Helmet allows you to regenerate HP in the late game without constantly recalling.
Tank Emblem
For Marcel's emblem, I recommend using the Tank Emblem to increase HP and defense. For the talents, choose Vitality to increase HP, Tenacity to reduce damage taken, and Concussive Blast to deal area damage.
Battle Spell
I suggest that you pay attention to your teammates' health points frequently, and go forward to heal them when you see them in danger so that they can continue fighting. Otherwise, they will easily be killed by the enemy.
Early game
Now let's move on to Marcel's early game. Although Maecel is quite squishy in the early game, his passive and first skill allow him to easily lock down enemies, giving him a good chance to escape. So don't play too passively. Try to help your teammates control enemies so they can secure kills more easily. This will help your team gain an advantage. Otherwise, if you just keep roaming around later without contributing, you won't be very useful to the team. When fighting for the Turtle, try to use your ultimate to lock down the enemy jungler so your teammates can rotate and take him down. This will make it much easier for your team to secure the Turtle. You should also rotate to mid lane frequently, because your movement speed and skill range make it easy to catch the enemy mid laner, allowing your teammates to secure the kill more easily.
Mid game
Next is Marcel's mid game. During this phase, Marcel should often stay with the team's jungler, because he is the strongest teammate at this time. Marcel needs to keep stunning or locking down enemies so the jungler has a chance to enter the fight and make a surprise attack. Marcel's ultimate can be used for two purposes. The first is to immobilize enemies, so your teammates know which target to focus on once the ultimate ends. The second is to save teammates who are being chased. Try to use your ultimate to lock down the enemies chasing your teammate, giving them time to escape. However, if the enemy is already too close, you can also stun your teammate together with the enemy, buying time for other teammates to arrive and help save them.
Late game
Finally, we move into Marcel's late game. During this phase, Marcel should stay close to the marksman(adc), because your shield becomes extremely important at this stage. Try to give your shield to the adc while they are in their power spike. In addition, you should save your ultimate for the enemy adc. If you can't stop them, your team will have a very hard time getting close. So try your best to lock the enemy adc inside your ultimate, giving your teammates the chance to eliminate them. Once the enemy adc is taken down, your team can wipe out the rest of the enemies and send them all back to Home for a coffee break.
Best With Vale
Marcel's ultimate allows him to temporarily trap enemies in one area. This is very helpful for teammates who are rotating in to support, especially heroes with AoE burst damage, because they can wait until Marcel's ultimate ends and then immediately wipe out the enemies. Examples include Vale, lxia and Hanabi.
Watch Out For Ixia
Marcel's ultimate allows him to temporarily trap enemies in one area. This is very helpful for teammates who are rotating in to support, especially heroes with AoE burst damage, because they can wait until Marcel's ultimate ends and then immediately wipe out the enemies.